Chinese new year is on random date in january. Make a variable than generates random date from 20 - 29 for the chinese new year. Who cares about other calendars really.
Jesus rose to heaven, decrease spawn time for creeps. No, it's when Petrus made 5000 people believe in Jesus(if im not wrong), so maybe make all creeps neutral.
On my birthday, create a unit called Death that will kill all units, creeps, heroes, but not invulnerable ones, if you want.^^
Jesus' born, is when 3 astronomers came to Jesus and gave jewels, gold, etc. So maybe increased bounty?
Apr 1st : April Noob, Creep drops will become fake ultimate items. e.g if Lightning Curiass give +300 armor, the fake one gives -5 instead.