The VOIDRUNNER'S CODEX is LIVE! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
I have no idea either, however I'm sure that the FRCS talks about the calendar year. I remember reading about it. Its very detailed, with names for days, months, holidays, etc.
There are a number of calendars out on the web for the Forgotten Realms. www.daggerford.com - my website - has an extensive list of FR sites, including a number of calendars.
I'm intending to write a new calendar soon with holy days included - but then soon can be such a long time!