I have done it twice.
For the first one, we started with a BIG poster board. We printed out pictures of attractions, cut them to a uniformed size and glued them on the board to form the calendar. Then I covered each picture with a blank piece of paper and then covered each piece of paper with Home Depot mickey paint chips with numbers written with a sharpie for the dates. Under each HD paint chip, was a trivia question. Each night, we would pull off a paint chip to reveal a trivia question and then we pulled the piece of paper off to reveal the picture which of course is the answer to the trivia question.
For the second one, we created the calendar in Powerpoint. We printed it out and attached a Disney pin to the back, one for each day. The pins were purchased on eBay. The backing of the pin (the mickey head) was visible on the front in the square for each day. The kids took turns removing a pin each day and added them to their lanyards for trading in the parks.