Not sure. Usually ending 5 mins after I log on 
.....never fails
They often occur randomly. The best time to form one is around 9-10pm est. Mike Monkey's been forming em every monday around 9pm est. if you're looking for a more regular time. Everytime i need the badges I just form them myself. If you start during a high populated time, people usually show up.
Triumph Lurker: mintmiki 50 emp/archer
basically, if you see a miki on Triumph, it's probably cute and it's probably me.
Huge thanks to cuppamanga and all the folks in the mac help forum for prolonging my borrowed time on this game.
When are they normally scheduled? I want to make some progress on the two raid badges.