Here's the list (don't know why i'm doing this):

February -
14th - Valentine's Day

March
17th - St.Patricks Day

April
1st - April Fools' Day - Celebrated each year with a new practical joke
23th - Star Day - A Star Syndicate Holiday
30th - Pico Day - Celebrates Newgrounds's Mascots from the Pico Series and Tom Fulp's Birthday

May
26th - Lock Day

July
10th - Robot Day
18th - Glock Day

August
15th - Clock Day - celebrating the passing through judgment of the movie "B", upon which the Clock Crew are founded. (also my birthday)

September
22nd - Madness Day - Celebrates the Madness Series

October
25th - Ban Day- Any Lock who doesn't make a submission for this day is banned from their forum.
31st - Halloween- Annual Celebrations, with competitions. There is a main Collection for Halloween and separate Collections for each year since 2000. On 30th October 2010, all user's level Pix were changed to a variety of Halloween Pix.

December
25th- Christmas