The Greyhawk year consists of 364 days, split into twelve months of twenty-eight days each, and four seven-day festivals, each placed at three-month intervals. Different cultures in the campaign setting have their own names for the months and festivals. The months and festivals are commonly referred to as the Dozenmonth of Luna and the Four Festivals.
Days in Greyhawk are reckoned in weeks of seven days each. The days of the month are: Starday, Sunday, Moonday, Godsday, Waterday, Earthday, and Freeday. Godsday is commonly associated with worship, and Freeday with rest. The remaining days are considered "work days."
An interesting feature of the Greyhawk calendar is that the days of the week always occur on the same day every year, a result of the year having a number of days divisible by seven.
The first of the month is always a Starday, and the rest follow as shown below:
Starday is always on the 1st, 8th, 15th, and 22nd of the month.
Sunday is always on the 2nd, 9th, 16th, and 23rd of the month.
Moonday is always on the 3rd, 10th, 17th, and 24th of the month.
Godsday is always on the 4th, 11th, 18th, and 25th of the month.
Waterday is always on the 5th, 12th, 19th, and 26th of the month.
Earthday is always on the 6th, 13th, 20th, and 27th of the month.
Freeday is always on the 7th, 14th, 21st, and 28th of the month.
The months and festivals are based on the cycles of Oerth's moons, Luna and Celene. Luna has a twenty-eight day cycle, while Celene's cycle is ninety-one days. Celene is full at the midpoint of each festival, while Luna is full at various times throughout the year. Notably, both moons are full on Richfest 3-5.