Leetmas is the time of year at Video Game High School where all holidays are celebrated at once. This include Christmas, Pi Day, Easter, Thanksgiving, Saint Patricks Day, Halloween, Independence Day, etc. This holiday is shown in Episode 5 of Season 2.
How Leetmas became-
Leetmas started a year or so before BrianD joined Video Game High School. In the beginning of the episode, you can here the ending of the story on the radio while BrianD is cooking. The story, by context clues, is that Calhoun tried getting rid of all celebrations of holidays with him being a character with a grumps personality. The students fought back and the battle of holidays ended with a compromise that is Leetmas. This is so that the students can still celebrate holidays and Calhoun only has to deal with it for one day.