I usually didn't try to get my kids excitement level up until we were only a few weeks away from our trip. I started our countdown chain only 30 days out, and less if the trip was during the school year. A trip is exciting, and a great thing to look forward to, but I never wanted that to diminish the excitement of a soccer game, a school awards night, a band concert, etc.
I think your kids will get amped up enough right before the trip to have that excitement carry right on through the entire vacation.