The Brief: On Reddit, Cake Day is the anniversary of when someone made their account.

DEEP DIVE
It’s like a birthday, but for Reddit. When it is someone’s cake day, a picture of a slice of cake shows up next to their username. Redditors are typically extra generous with their upvotes when it is someone’s cake day. A number of memes about cake day reference the surge in upvotes and Reddit karma that people can get on their cake day.
Only because it’s my cake day, I decided to show my kids dad used to be cool. 1988 cool. from r/OldSchoolCool
When you forget today is your cake day and find out at 11pm. from r/PrequelMemes