If your Japanese is up to it (or can get help) this is a very user friendly nation wide cosplay event calendar. It's only flaw is that sometimes events get posted late. Like the TFT Tonaride Cosplay event isn't listed yet for some reason. Just check the dates you'll be in Tokyo and see if anything is going on. Heck, check for where ever you are at the moment, might get lucky.
Archives has a more comprehensive list, but is more difficult to navigate.
Summer Comiket 90: Tokyo, Japan
Military Odyssey: Detling, England