On Sep. 28, 2009, MathOverflow went online, and has almost ten years of almost constant operation, thanks to StackExchange.
There will be an obligatory post observing the anniversary. The board of MathOverflow LLC might hold a toast (with coffee and tea) in observance as well, and they may yet have bigger plans. Until those plans appear, this post is a staging area for people who want to celebrate the forum away from the forum (as in meeting in some locale). For a timely and well anticipated event, starting plans about now seems good.
How do you want to observe the anniversary? And where, and who do you want to invite?
Gerhard "Where Are All The Parties?" Paseman, 2019.06.21.