If I was making a fantasy calendar, I would take the month of the greatest celebration in the land and call that month "Komodober." All other months would receive names that would reflect their status as "Boring," "Stupid," and "Not Komodober."
All joking aside, I actually did go through some lists of old calendars. Here's some of the more interesting-sounding names I found (with some heavily Romanicized spelling for some of the older ones):
- The german word for January is spelled "Januar" but pronounced "Yan-oo-arr"
- Before Julius and Agustus Caesar named the calendar after themselves, the Romans used the names "Quintilis" and "Sextilis" for what we know as July and August.
- Hornung and Weinmonat (Feb. and Oct. in 15-18 century Germany)
- Ostarmanoth (April in Germany prior to the 15th century)
- Yuányuč (Chinese, means "first month")
- Qiū tian (Chinese, means "Autumn")
- Sukurrizis ("Garlic-collecting month," June-July in ancient Persia (modern-day Iran))
- Xordād (also Persian, means "wholeness," not sure what time of year)
That's just the names I found simplest and/or most memorable. However, along with that, let me give you a bit of perfectly unsolicited advice: find a way to incorporate the calendar into the campaign world, and put the connection in a place the players/readers will see it. For example, great emperors tend to name important stuff (like months) after themselves, so add that particular piece of lore into the worlds backstory, with repercussions that touch the characters to this day. Or have an arc in garlic-collecting-month in which the characters encounter a serious garlic- related emergency (shortage for the local feast? Swarm of vampire rabbits?). What I'm saying here is, if you're going to name the months, then make them significant to the players somehow. Otherwise, it'll just be a list of hard-to-pronounce names that no-one will take the time to remember.