Eh, I posted a "Hitchhiker's Guide to Britain" for my 1000th post, simply as a bit of fun, but the idea came first and then the thought of doing it for my 100th came afterwards when it was nearing completion.
That said, I meticulously planned my 10K/Escapist anniversary celebration for about three months prior to it, and then everybody suddenly hit 10K at the same time as me, making it less special, but the fact it was a 1 year thing too made it less dull.
I probably won't do anything for 15K but probably will for 20K, and then my 2 years will come before 30K, but I'll probably celebrate both, and I have no idea how long I'll be around after that.