Perhaps I should explain myself more.
Even though Yliakum is not supposed to be related to 'real world' in any sort of matter, we still have apples, fish, swords, forges, books, librums, guards...etc that are borrowed from the 'real world'. Slight perhaps, but nevertheless they are still borrowed. Therefore, a logical conclusion would be for the religion(s) or races of Yliakums to have days of feasts/fasts/worship that serve to break up the year into smaller parts.
Although I agree that the feasts/fasts/worship days should not be in any way directly or indirectly related to 'real world' holidays (for instances, having a Christmas or easter like celebration would not make sense in Yliakum), I think that the feasts/fasts/worship days should fall around the dates of such events in the 'real world'...just to provide an IC explanation for the player's merriment and/or reflection of such merriment in their characters.
Besides....it would be great for Role play; something different than fighting and slaugtering and running around completing quests.