I don't have the dates for the Carlton Ornaments, but normally they are put out sometime in July. They don't have a big premiere event like Hallmark does.
The Hallmark premiere is on July 12 and 13th. "On Frozen Pond" (the Hallmark name for the Kermit and Piggy ornament) will be available on that day, while supplies last.
A lot of times, Hallmark does sell out of some of thier ornaments on opening day, but they almost always get more in thruout the season. I don't see this being terribly hard to get this summer and fall.