So is this going to be sent to anyone who registered? If not this week (a little late perhaps), then in future weeks? (or maybe it already was)
I would adjust the messaging a little to make it clear what exactly this is (and that the event is not in Potomac):
Subject: TEDx Tuesdays - New Happy Hour for TEDxPotomac
Please join TEDxPotomac for its new TEDx Tuesdays Happy Hour, to be held every week from now until the big event on May 20! (possibly something about purpose, program, etc. for the HH)
[Details - as above, except for confusing name]
Paul