I am trying to drop an OCS packet (AC). Where can I find out the panel dates for FY 2020?

Here is a link to all the board dates, which includes OCS:
https://www.public.navy.mil/bupers-npc/career/careercounseling/Documents/FY20%20Board%20Schedule%20Final.pdf

These are listed by community, and you need to read between the lines. For instance, CEC has many boards in the list, but since the vast majority of CEC come in via OCS, you can surmise that quotas in the quota boards will be mostly for OCS.

Search MILPERs on HRC website. The MILPER is very descriptive on wlcreitieria, dates, etc.

The OCS dates are announced in a MILPER message every year, as well as the selectees.