Certain Fridays in which your caps lock must be on all day. There is no real method in decided when it's Caps Lock Friday, but some people practice Caps Lock Friday every Friday.
(Unfortunately Urban dictionary does not allow a certain amount of capital letters in an example, so just imagine everything in capital letters)
Guy1: Hey did you hear?! It's Caps Lock Friday!
Guy2: Are you serious? I guess my term paper will have to partly capitalized.