Thursday at Emerald Tavern from about 7pm - midnight is the regular day for casual play. ET is, unfortunately, inconsistent about running GNK events. But for that we have a regular GNK night at Dragon's Lair on the second Wednesday of each month.
Doomrat is correct in that most sudden playgroups or random tournaments are handled via Facebook. That said, if you're attending the regular Thursday meetup you'll definitely hear of any surprise GNK or bigger tournament events.