First thing first: any calendar in any world is going to be based on something obvious - usually astronomic, like ours is. So, designing your astronomy logically comes first, before designing your calendar.
In my first big game (Telenet) I used conventional earth months etc., except they all had 30 days. Easy to grok for all involved, but boring as hell, particularly since most of us had come out of a game that used a Tolkenian Elvish calendar (6 months each of 60 days).
Riveria had two moons. Most people based their calendars off the faster one, that went through its phases 13 times a year; hence 13 months of 28 days each, and that's what was used as the standard with the months names not related in the least to real-world. The Elves, however, based their calendar off the slower moon, which ran at exactly half the rate of the faster - so the Elvish "year" was 2 solar years long, made up of 13 months of 56 days each. (a friend and past/current DM with a masters in astronomy tells me I blundered onto something that is in fact quite realistic here...two moons tend to settle into orbits mathematically related to each other, and 2-to-1 is very possible) Different cultures used different days to represent New Year; this didn't work out very well.
Decast will try something different again. Once more we have 2 moons, only this time they're on a 4-1 orbit ratio - Fieros has a 20-day cycle; Stabos takes 80. So, the standard calendar will use a double cycle of Fieros for its months - 40 days - and there'll be 9 of 'em. I'll use Elvish names modified from JRRT, with a few new ones tacked on; thus Hriva, Tuila, Laire, Yavia, Quelle, Coira, Auril, Eolna, Flaige. Other cultures have different names for the months, but the Elvish will be standard. The day after winter solstice (Hriva 1) will be New Year for everyone.
Holidays - the pagan Wheel of the Year (solstices, equinoxes, cross-quarters) makes the most sense here, again because it is astronomically based and thus similar to all surface-based cultures. I've changed some of the names for fluff reasons - the one great Gnomish contribution to my worlds, for example, is the holiday of Skandigan (formerly Lammas); a day during which any self-respecting Gnome and an awful lot of non-Gnomes get roaring drunk and accomplish little else of use. Naturally, local areas can have holidays whenever it makes sense for the story - these are just the major ones here.
Each player gets a player's kit on joining the game, containing among other things a 1-page write-up on calendars and holidays.
Lanefan