It's exactly what it looks a like, a save the date postcard! One of the people who attended the meet up sent these to everyone who came. She quickly got my address before I left and I found this in my mailbox on the day I arrived home.
Received: Between Feb 8-26
The stamps were arranged in a nice way, and the postmark is even better as it shows the venue for the meet up. There was a post office located there, which was a pleasant surprise to me (though it was closed when I passed it by at 7 am...)