well, i think it should be only events that are "normal" for most countries, for example 4th of july only applies for the united states and the rest of the players from other countries might know about that day but they probably dont celebrate it (like myself), but it could be changed into a version that celebrates a special day of the town in game - call it i dont know... founding day or whatever... just some opinions
edit: i know that it wouldnt be that many holidays in that case since the countries differ with their holidays, but at least the game would have the major ones - easter, christmas and new year
and lets say the game features the above mentioned founding day as a holiday, maybe add events for the summer and winter solstice... i dont know...