From the Forgotten Realms Campaign Setting book:
Almost every people or race of Faerun marks the passage of days, seasons, and years in some fashion. In Cormyr and a dozen other kingdoms, royal astrologers carefully tend to the Roll of Years. Even the war-heralds of the unlettered orc-tribes compose harsh chants that record the days and deeds of their fierce chieftains.
Faerun’s days are 24 hours long, divided into night and day by the rising and setting sun. In southern lands such as Halruaa, the length of the night does not vary much with the season, and 12 hours of light and 12 hours of dark is the rule year-round. In the north, the days are markedly longer in summer and shorter in winter. Midwinter say in Silverymoon sees little more than 8 hours of daylight, and Midsummer almost 16.
Ten days comprise a Faerunian week, also known as a tenday or less commonly, a ride. The individual days of a tenday do not have names. Instead they’re referred to by number: first-day, second-day, and so on. Most folk start counting using their thumb as first day, but halflings are famous for using their pinkies to count first-day, so much so that the phrase “counting like a halfling” means that someone is being different just to be difficult.
Timepieces are very rare, and most people break up the day into ten large slices – dawn, morning, highsun (or noon), afternoon, dusk, sunset, evening, midnight, moondark (or night’s heart), and night’s end. Dozens of conventions for naming these portions of the day exist, and cause no little confusion for travelers in foreign lands.
These customary divisions are only approximations, and one persons late afternoon might be another’s early dusk. Local customs dictate the general length of each portion of the day. Each of these customary periods last anywhere from 1 to 4 hours, so highsun is generally accounted to be noon and an hour or so on either side.
Few Faerunian’s have cause to measure an hour (or any length of time shorter than a day) with any great precision. People are accustomed to gauging time by intuition, the movement of the sun, and the activity around them. Two merchants might agree to meet at a particular tavern at dusk, and chances are both will show up withing 15 or 20 minutes of each other.
In large cities, the tolling of the temple bells replaces the more casual accounting of the day’s passage. Several major faiths attempt to measure time more accurately. The priests of Gond treasure their mechanical clocks and delight in sounding them for all to hear. Lathanderians assign acolytes to watch sundials, carefully adjusted by years of observation of the sun’s movements in the sky. Traditionally, the hours are numbered 1 to 12 twice, and the bells sound once for each hour on the hour. “Twelve bells” is virtually interchangeable with “midnight” or “highsun” depending on the context.
Most of Faerun uses the Calendar of Harptos, named after the long dead wizard who invented it. Few bother to refer to Harptos by name, since the calendar is the only calendar they know. Each year of 365 days is divided into 12 months of 30 days, and each month is divided into three tendays. Five special days fall between the months. These annual holidays mark the seasons of the changing of the seasons. The months of Faerun roughly correspond to the months of the Gregorian calendar.
- Month 1 = Hammer or Deepwinter
- Annual Holiday: Midwinter
- Month 2 = Alturiak or The Claw of Winter
- Month 3 = Ches or The Claw of Sunsets
- Month 4 = Tarsakh or The Claw of Storms
- Annual Holiday: Greengrass
- Month 5 = Mirtul or The Melting
- Month 6 = Kythorn or The Time of Flowers
- Month 7 = Flamerule or Summertide
- Annual Holiday: Midsummer
- Month 8 = Eleasis or Highsun
- Month 9 = Elient or The Fading
- Annual Holiday: Highharvestide
- Month 10 = Marpenoth or Leaffall
- Month 11 = Uktar or The Rotting
- Annual Holiday: The Feast of the Moon
- Month 12 = Nightal or The Drawing Down