I think you're thinking of several different conventions, all of which are hosted by HMGS-East.
Historicon has usually been in Lancaster, but was slated to move to Baltimore starting in 2010. Something went down with the contract in Baltimore, so they moved it to Valley Forge instead.
Cold Wars is usually held in Lancaster.
Fall-In has usually been held in Gettysburg, but with Histoicon moving out of Lancaster, Fall-In is slated to move to Lancaster.
I usually go to the ones in Lancaster (and I've been to Gettysburg once before, and would have liked to go again.) But this year I'm only able to go to Cold Wars, which was just a couple weeks ago. All of them are excellent conventions, with a lot of games, dealers, flea market, etc.