First, a bit on the stars in the sky and other heavenly things:
There are two moons. Luna’s cycle is 28 days, or one Greyhawk month long. Celene’s cycle marks the seasons of Greyhawk, lasting 3 months and one festival week that separate each season. Here’s a full calendar with the moon cycles and the feast days of various deities.
The deities of Greyhawk are many and not so far removed from the world – calling upon them could have results beyond your cleric’s cure spells. Here is a webpage that lists the deities. Another source with more game related info are the pdfs linked in this document.
Locally, within Fort Talmadge there are two temples. The main one serving the town is the temple of Pelor, lead by the Priestess Clamella. Also in town is a small shrine to Farlangahn, served by traveling priests.
In Greyhawk, the most popular and influential priesthood is that of Saint Cuthbert