What was the most significant event on April 15 during the U.S. Civil War updated?

To me, the most significant event on April 15 during the Civil War was the death of President Abraham Lincoln
My response:
1865: On the morning of April 14, 1865, President Lincoln woke in an uncommonly good mood. On Palm Sunday, April 9, Robert E. Lee had surrendered to Ulysses S. Grant. The truce reached at the Appomattox, Virginia, Court House signaled the end of the Civil War. To celebrate, Mr. and Mrs. Lincoln decided to attend the hit farce comedy “Our American Cousin,” which was playing at Ford’s Theatre.
After the President had died on April 15, Secretary of War Stanton saluted the fallen president and famously uttered, “Now, he belongs to the ages.” (Some have argued that Stanton said “Now, he belongs to the angels.”) Stanton further eulogized President Lincoln with the apt observation, “There lies the most perfect ruler of men the world has ever seen.”

Since RallyPoint truncates survey selection text I am posting events that were not included and then the full text of each survey choice below:

A. Monday, April 15, 1861: President Lincoln calls for 75,000 volunteers for three month’s service to deal with the "insurrection." The slave states still in the Union react negatively. The states of Kentucky and North Carolina both issue statements refusing to raise troops for Federal service.
B. Tuesday, April 15, 1862: The Battle of Peralta was a small battle near the end of Brig. Gen. Henry H. Sibley's New Mexico Campaign. Retreating from the Battle of Glorieta Pass, Confederate troops of the 5th Texas Mounted Volunteers, under Col. Thomas Green, camped in the town of Peralta and planned to cross a series of irrigation canals the next day.
On April 14, Green found himself near the Governor's Connelly's house at nightfall and decided to camp there and cross the following morning. Peralta was a 2-mile stretch of adobe houses, thick adobe fences, raised ditches, and groves of large cottonwood trees. Connelly's residence, where Green had set up his temporary headquarters, was surrounded on all sides for a half mile by a low, heavy growth of trees. Only the main road leading to the house could be used for wagons or artillery. Numerous wide irrigation canals ran parallel to, and across the road, while adobe walls enclosed the fields.
On April 15, The Union troops assumed that there would be an immediate assault by Confederate infantry and cavalry, supported by the artillery posted to the east and north of Connelly's house. Several Confederate cannons were within this maze of fields. The pursuing Union forces under Col. Edward Canby caught up to Green.
The Federals attention was diverted to the north, however, where a Confederate supply train approached Peralta from the direction of Albuquerque. Consisting of 7 heavily laden wagons, the train was escorted by a detachment of Texans with a mountain howitzer. Canby attacked the supply train and the Confederates had to stop and defend themselves where the Union troops charged to within 50 feet of the wagons and cannon.
At midday, Canby sent separate columns under Cols. Gabriel R. Paul and John Chivington around to the north and west of Peralta to surround the Confederates to prevent any more forces from joining Green. Outnumbered and outgunned, Green stuck to his position in and around the mansion and fields. The two armies battered each other with artillery until a dust storm blew in at about 2:00 P.M. The Confederates headed out to the southwest and ended the battle. They left behind a shambles of buildings and groves in and north of Peralta.
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C. Wednesday, April 15, 1863 --- Walt Whitman, the poet, writes to his mother in Brooklyn from Washington, where he spends his days looking after wounded and ill soldiers: I spent three to four hours yesterday in Armory hospital. One of my particular boys there was dying—pneumonia—he wanted me to stop with him awhile; he could not articulate—but the look of his eyes, and the holding on of his hand was deeply affecting. His case is a relapse—eight days ago he had recovered, was up, was perhaps a little careless—at any rate took cold, was taken down again and has sank rapidly. He has no friends or relatives here. Yesterday he labored and panted so for breath, it was terrible. He is a young man from New England, from the country. I expected to see his cot vacated this afternoon or evening, as I shall go down then. Mother, if you or Mat was here a couple of days, you would cry your eyes out. I find I have to restrain myself and keep my composure—I succeed pretty well.
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D. Saturday, April 15, 1865: President Abraham Lincoln dies in a boardinghouse opposite Ford's Theater. Vice President Andrew Johnson takes the oath of office and became the 17th president of the United States of America.
At 6:50 a.m., Dr. Charles A. Leale recorded what he observed: “The respirations cease for some time and all eagerly look at their watches until the profound silence is disturbed by a prolonged inspiration, which was soon followed by a sonorous expiration. The Surgeon General Joseph K. Barnes now held his finger to the carotid artery, Col. Charles Crane held his head, Dr. Robert Stone (the Lincoln’s family physician) who was sitting on the bed, held his left pulse, and his right pulse was held by myself.
“At 7:20 a.m.,” he wrote, “he breathed his last and (here, Leale paraphrases Ecclesiastes 12:7) ‘the spirit fled to God who gave it.’” (Most historians give the time of death at 7:22 a.m.)
More famously, Secretary of War Stanton saluted the fallen president and famously uttered, “Now, he belongs to the ages.” (Some have argued that Stanton said “Now, he belongs to the angels.”) Stanton further eulogized President Lincoln with the apt observation, “There lies the most perfect ruler of men the world has ever seen.”

1. Tuesday, April 15, 1862: Union naval forces enter the mouth of the Mississippi and sail up the river to Fort Jackson.
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2. Tuesday, April 15, 1862 --- An artist named John Henry Brown writes in his journal: "Petitions have been sent to congress praying that the same mail facilities may be extended to Democratic papers which are enjoyed by Republican papers. This looks strange in this land of liberty, the Constitution of which declares, that the freedom of speech and of the press shall not be abridged. The truth is, to be a Democrat now is to be a Traitor, in the estimation of the Administration."
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3. Tuesday, April 15, 1862 --- The New York Times publishes this eulogy for Albert Sidney Johnston, the Southern commander at Shiloh: The death of the rebel General, SIDNEY JOHNSTON, at the battle of Pittsburgh, is now definitely settled; and the rebels grievously mourn the loss of their distinguished commander. He had previously been under a cloud on account of his want of success in Kentucky and Tennessee; but he vindicated both his courage and skill, as well as the sincerity of his treason, on the fatal Sunday upon which he fell; and though that affair, like all his previous efforts in this war, was a failure, yet rebel criticism is silent in presence of his death. It was undoubtedly he, and not BEAUREGARD, who conceived and managed the brilliant rebel maunoeuvering of Sunday, by which the rebel columns were shifted with such rapidity, and hurled now upon one of our wings, then upon the other, then upon the centre, then upon some apparently weak point, and everywhere with a spirit and dash that could not be excelled. The tactics were all characteristic of JOHNSTON. He exposed his person, too, with recklessness. It is probable indeed that he had been driven to desperation by the assaults that have recently been made upon him, and that in despondency and misery, he courted death upon the field. It is a sad ending to a career which had for so long time been eminent and honorable.
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4. Tuesday, April 15, 1862 --- Mary Boykin Chestnut notes in her diary, after a list of losses and ills that the South is suffering: "I would rather live in Siberia, worse still, in Sahara, than live in a country surrendered to Yankees."
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5. Wednesday, April 15, 1863 --- Lt. Col. Arthur Fremantle notes in his journal his meeting Gen. John Magruder, commander of the Department, as he travels across Texas: I slept well last night in spite of the tics and fleas, and we started at 5.30 p.m. After passing a dead rattlesnake eight feet long, we reached water at 7 a.m.
At 9 a.m. we espied the cavalcade of General Magruder passing us by a parallel track about half a mile distant. . . . I galloped up to the front, and found the General riding with a lady who was introduced to me as Mrs ——, an undeniably pretty woman, wife to an officer on Magruder's staff, and she is naturally the object of intense attention to all the good-looking officers who accompany the General through this desert.
General Magruder, who commands in Texas, is a fine soldierlike man, of about fifty-five, with broad shoulders, a florid complexion, and bright eyes. He wears his whiskers and mustaches in the English fashion, and he was dressed in the Confederate grey uniform. . . . He is a Virginian, a great talker, and has always been a great ally of English officers. . . . I had a long and agreeable conversation with the General, who spoke of the Puritans with intense disgust, and of the first importation of them as "that pestiferous crew of the Mayflower;" but he is by no means rancorous against individual Yankees. He spoke very favourably of M'Clellan, whom he knew to be a gentleman, clever, and personally brave, though he might lack moral courage to face responsibility. Magruder had commanded the Confederate troops at Yorktown which opposed M'Clellan's advance. He told me the different dodges he had resorted to, to blind and deceive the latter as to his (Magruder's) strength; and he spoke of the intense relief and amusement with which he had at length seen M'Clellan with his magnified army begin to break ground before miserable earthworks, defended only by 8000 men.
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6. Friday, April 15, 1864: The Skirmish at Camden on April 15, 1864, occurred after Union brigadier general Frederick Steele had forced Major General Sterling Price’s troops and cavalry out of Camden (Ouachita County) on April 12. Realizing his opportunity, Steele marched his army approximately forty miles to the east toward Camden. This would prove to be an important turning point within the Red River Campaign for the Union troops.
In the early hours of April 15, the Thirty-third Infantry of Iowa began its march toward Camden, still eighteen miles away. Its first movement on the Confederate lines forced a battery on the main road to cease firing, allowing the troops to continue advancing toward the city. By 10:30 a.m., the Thirty-Third Infantry had marched through skirmishes for roughly three hours. These skirmishes would slow down the advancing troops, but the Confederate forces knew it would be impossible to prevent Steele’s army from entering Camden. Nearing the end of the skirmishing, the Confederates realized it would be necessary to move south in order to avoid becoming pinned down within the city.
During Steele’s march, Maj. Gen. Price’s cavalry pursued, attacking the front and rear of the forces in an attempt to halt Steele’s entrance into Camden. Although a nuisance to the Union forces, the attack ended in failure, stemming from Price’s quick and uncoordinated effort. The delay, however, was a psychological victory for the Confederacy. By the afternoon of April 16, the entire Union brigade entered the town, having complete control.
Low morale and food shortage hampered the Union troops as they entered the town. It even became necessary to forage as food supplies dwindled in camp. The delay cost the Union troops and made it nearly impossible to venture too far from their fortifications. Prior to their arrival, Steele requested thirty days of rations for his troops set to arrive at Camden by April 15. The Adams and Chippewa, the steamers sent to provide supplies, did not meet their deadline. The Adams sank close to its home port, while the Chippewa sustained damage and returned home. In town, Camden residents “dumped animal carcasses in many local wells,” practically eliminating any means of having drinking water for the troops. At the same time, Union troops knocked on doors, demanding food from all Camden residents. Steele hastily stepped in, halting his troops from pillaging. It would be a few days before any supplies arrived, leaving Steele and his men to conservatively utilize supplies and attempt foraging expeditions.
While the Union dealt with poor morale, troops established a light fortification line around Camden with the hopes of keeping the Confederacy at bay until their supplies arrived. Steele needed to contact Union general Nathaniel Banks, who was leading the column in northwestern Louisiana, with the understanding that he would not be falling into a trap by fortifying his troops within the city. Meanwhile, Lieutenant General Kirby Smith provided a morale boost for the Confederates as he began his move of three infantry divisions to Camden.
The true fighting would occur on April 16–18, 1864, as Steele realized the need to attack the Confederate troops before reinforcements could arrive. April 15, however, laid the groundwork for an important series of skirmishes during the overall campaign.
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7. Friday, April 15, 1864 --- Gen. Steele, with his Yankee forces now safely in fortified Camden, Arkansas, is not cheered by Sherman’s repeated order that Steele should venture forth into Louisiana to take Shreveport. But Steele answers in his own message, pointing out Marmaduke’s cavalry division that he has been sparring with, plus another cavalry division moving in from the west. Had he known it, he could have named yet another 3 divisions of infantry headed his way from Shreveport under Kirby-Smith.
Sherman decides, in spite of the unfinished business in the Red River Expedition, not to delay any longer, and he summarily orders Smith to leave Louisiana and go to Chattanooga. However, Banks countermands that order, insisting that he cannot do without Smith’s divisions.
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8. Friday, April 15, 1864 --- Banks’ troops and the accompanying fleet arrive in Grand Ecore, Louisiana, closely pursued by the Confederate army. They resume the retreat the next day toward Alexandria.
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9. Friday, April 15, 1864 --- The USS Eastport, the heftiest ironclad in Porter’s Red River flotilla, strikes a river torpedo and sinks. Operations begin immediately to raise the vessel.
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Abraham Lincoln Coffin Photo: The Crazy Story Behind It
In all the years since Abraham Lincoln's death in 1865, just one photo of Lincoln after his death (in his casket) has emerged. Despite millions of people witnessing Lincoln in his coffin, Mary Lincoln had given specific instructions that no photographs were to be taken. However, a photo was taken in New York City, and it wasn't discovered until 1951! This is the story of the Lincoln coffin picture.
https://www.youtube.com/watch?v=5RYHv07T4IY

Ive visited Ford's Theater and the house across the street where President Lincoln was shot and where he died several times. Always an emotional time to be part of that history. Sad indeed. Thank you for sharing LTC Stephen F. , as always.

I would have to say the Death of Abraham Lincoln as it is still a relevant topic in general.....A close second probably the call for volunteers.