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January 15 is known as Wikipedia Day to Wikipedians. On this date in the year 2001, the wiki-based Wikipedia project went public after spending five days on Nupedia.

Wikipedia Day has been celebrated, both online and offline, for several years. In 2011, there was a big worldwide celebration of Wikipedia 10, and this tradition continued with diverse local meetups and wiki-events for Wikipedia Day on Sunday 15 January 2012.

The earliest known proposal for an online encyclopedia was made by Rick Gates in 1993, but the concept of an open source web-based online encyclopedia was proposed by Richard Stallman around 1999. Wikipedia was formally launched on 15 January 2001 by Jimmy Wales and Larry Sanger, using the concept and technology of a wiki pioneered in 1994 by Ward Cunningham. Initially, Wikipedia was created to complement Nupedia, an online encyclopedia project edited solely by experts, by providing additional draft articles and ideas for it. Wikipedia quickly overtook Nupedia, becoming a global project in multiple languages and inspiring a wide range of additional reference projects.

As of January 2013, Wikipedia includes over 24.1 million freely usable articles in 285 languages, written by over 38 million registered users and numerous anonymous contributors worldwide. According to Alexa Internet, Wikipedia is the world's sixth-most-popular website, visited monthly by around 12.8% of all internet users.