The Rice University Neologisms Database

Analogous to birthdays, the day of the year on which someone created their account, specifically their username, for a website. Though the user usually is familiar with the website before getting an account, creating and recieving a username is seen as becoming a full-fledged, active member of the website, and on a user's "cakeday" they celebrate website and their experiences on it. The practice began on reddit.com, but has since spread to other online communities. Often confused with birthdays.