Blogging, that is. The first post on this blog was Another head? (1 May 2006), and the second, Why 'Chess for All Ages'? (2 May 2006), established the routine for daily blogging. The post for the first anniversary was M'aidez, M'aidez (1 May 2007). I don't remember if I picked May Day on purpose or if it just worked out that way.
Google image search on 'May Day'
Blogger.com, which is the Blogspot.com tool for managing the blog, says there have been 2188 posts before this one. I doubt there will be a 20th anniversary, but who really knows? It's always one day at a time.