Recently read this book "Hezar sal nasr-e Parsi" ("A thousand years of Persian Prose"), one of the books discussed, was about ancient Persian celebrations and festivities. I shall go back and will look into it, hopefully I'll give you more information shortly.
In time, it seems, the number of ceremonies dwindled from a few per month, to a few per year, today (the origin to these ceremonies and celebrations were pre-Islamic).
t would be very interesting. There are at least twelve celebrations, one per month. The days of the months used to have names and there would be a celebration when the day & the month name coincide. No-Rooz and Mehregan are two that are still alive.
Dear Novel Writer:
Here are some names common in the 5th century Persia.
Please choose from the list. You may also find
http://www.vohuman.org/ as a good resource.