Calendar of Greyhawk

The Common Year

The Common Year is the most widespread system of reckoning in the in the Flanaess.
The 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 have their own names for the months and festivals. The "Common" month names are the ones typically used today in the central Flanaess and Obsidian Bay.
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.

Month	Common	Elven	Nomads	Season
Needfest
1	Fireseek	Diamondice	Tiger	Winter
2	Readying	Yellowillow	Bear	Spring
3	Coldeven	Snowflowers	Lion	Spring
Growfest
4	Planting	Blossoms	Frog	Low Summer
5	Flocktime	Violets	Turtle	Low Summer
6	Wealsun	Berrytime	Fox	Low Summer
Richfest
7	Reaping	Goldfields	Snake	High Summer
8	Goodmonth	Sunflowers	Boar	High Summer
9	Harvester	Fruitfall	Squirrel	High Summer
Brewfest
10	Patchwall	Brightleaf	Hare	Autumn
11	Ready'reat	Tinklingice	Hawk	Autumn
12	Sunsebb	Lacysnows	Wolf	Winter

Days of the Week

The days 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. (Work) Sunday is always on the 2nd, 9th, 16th, and 23rd of the month. (Work) Moonday is always on the 3rd, 10th, 17th, and 24th of the month. (Work) - Exact moon phase at first midnight Godsday is always on the 4th, 11th, 18th, and 25th of the month.  (Worship) Waterday is always on the 5th, 12th, 19th, and 26th of the month. (Work) Earthday is always on the 6th, 13th, 20th, and 27th of the month.  (Work) Freeday is always on the 7th, 14th, 21st, and 28th of the month.  (Rest)

Year, Seasons, and Celestial Bodies

The Oerdian year has 336 days, divided into 12 months of 28 days each. Solar and lunar months coincide, with the large moon, Luna, having a period of 28 days and the small moon, Celene, having a period of 84 days. Luna's new moon is always at the change of the months, while Celene's new moon is at the beginning of each season.
There are eight major astronomical festivals of the the year, four solar and four lunar. The solar festivals are the winter solstice, spring equinox, summer solstice, and autumn equinox. The lunar festivals are when Luna and Celene are full at the same time and happen exactly 42 days after the solar festivals. At each solar festival, both moons are new.
Planets There are also five visible planets, named Atroa, Sotillion, Telchur, Wenta, and Velnius. Atroa is the innermost planet and visible at the spring solstice, Sotillion the second planet and visible at the summer equinox. Telchur is the forth planet (Oerth being the third) and most visible around the vinter solstice. Wenta is the fifth planet, most visible in autumn. Velnius is the sixth and final planet, barely visible from Oerth and not associated with any season.
All celestial bodies are named after the gods they correspond to, tough not all people recognize them as such.
Seasons and Climate There are four universally recognized seasons. Their effects vary widely by geographical area. In general, the north and south of the Flanaess has easterly winds, warm in the south and cold in the north, both humid and bringing a lot of rain and snow and making the climate relatively stable. There are westerly winds and currents to the north and south of the Flanaess bringing warm water to the Dravmidj and Solnor Oceans. The weather in the Thillonieian peninsual is particularily violent and snowy, with a warm sea current from the east and cold winds from the west.
Inland areas have a more variable climate and are much drier. Summers days are warm, winter nights are icy cold even far to the south.
Autumn Dominant easterly winds brings rain and snow all over the Flanaess.
Winter When the winds let up, the cold comes to the north. Sudden storms are common, though the weather is generally fairly sunny. Winds tend to be from the north and north-east.
Spring Westerly winds clean up the skies, tough this is a season of great instability and there are generally a few easterly rainstorms, especially in coastal regions. These rarely blow very hard or very long.
Summer Weak south-westerly winds bring dry, warm air, perfect for maturing cereals.
Months and Weeks Each month consists of four weeks and is both 1/12 of a year and one cycle of the larger moon, Luna. Luna is in new moon at the beginning of each month and full on the 14th of each month. The new day begins at midnight.
Tides The tides are cyclic over the month. At the beginning of the first and third week, high tide is at dusk at dawn. Depending on the latitude and the time of year, the actual time of sunrise varies, but high tide occurs at 6 am and 6 pm at the beginning of each month. At the beginning of the second and fourth week, high tide is three hours later.