September 26 was when I posted it on devArt, no? X3
It depends on what you want to see as anniversary, the first playable version (featuring the previous characters) was posted on June 6 (which I have since replaced with the current version). If you go by start of development, that was on April 18.
Until now I didn't really celebrate the anniversary, but if I have a couple of ideas (and time!), I'm not opposed to that. Though, there's also Halloween and Christmas around that time to plan for. ;P