Hi!
We did not do a calendar, we did a chain link chain using construction paper. We numbered each one, and wrapped it around our hall way. Every day the kids alternated taking a chain off. It was a great way for them to participate and actually see the countdown. We kept the milestones (100 days, 50 days, etc) and saved them for our scrapbook and dated them. It was a lot of fun. Can't wait to do it again ourselves.
Good luck finding a way to countdown to your vacation!
-Shannon