How to create a "Trip" in the collections?

We just got back from a family vacation, and I'm organizing our pictures in the Photos app. At first it showed up in "Trips", which was great. But then it suddenly disappeared. I had been playing with the metadata on some photos (including adding/adjusting location) and thought maybe that had messed it up, so I selected all the photos and changed the location to the right city. Still not there. So then I said "revert to original location" thinking my changes had messed it up, but it's STILL not there. But it WAS. I don't get it. Yes, I know I can just make an album. But I'm trying to figure out why it WAS in the Trips section, then disappeared.