September 15th:
Game time of September 15, you can see a red plane flying overhead to 
celebrate the release date of Zoo Tycoon. 

October 31st:
Game time of October 31st, you can see a witch over your zoo. Check your 
scenery list, there will also be a Jack-O-Lantern available to buy.
On your desktop clock, of October 31st then all guests and buildings will 
have a black and orange theme

December 25th:
Game time of December 25th, you can see Santa Clause and his reindeers flying 
over your zoo. Check your scenery list, a snowman will be available to buy.