The Discordian or Erisian calendar is an alternative calendar used by some adherents of Discordianism. It is specified on page 00034 of the Principia Discordia.
The Discordian year 1 YOLD is 1166 BCE. (Elsewhere in the Principia Discordia, it is mentioned that the Curse of Greyface occurred in 1166 BCE, so this is presumably the start-date of the calendar.) As a reference, 2008 CE is 3174 YOLD (Year of Our Lady of Discord). While the abbreviation "YOLD" isn't used in the Principia, and the phrase "Year of Our Lady of Discord" is only mentioned once, it is a Discordian tradition to use that designation.
The Discordian calendar has five 73-day seasons: Chaos, Discord, Confusion, Bureaucracy, and The Aftermath. The Discordian year is aligned with the Gregorian calendar and begins on January 1, thus Chaos 1, 3173 YOLD is January 1, 2007 Gregorian.
The Erisian week consists of five days: Sweetmorn, Boomtime, Pungenday, Prickle-Prickle, and Setting Orange. The days of the week are named after the five basic Discordian elements, Sweet, Boom, Pungent, Prickle, and Orange. There are 73 of these weeks per year.
Every fourth year on the Discordian calendar, starting in 2 YOLD, an extra day is inserted between Chaos 59 and Chaos 60 called St. Tib's Day. This is because 4 years + 1 day = 5, a holy number, but the Discordian leap year also coincides with the Gregorian one. The result of this is that any given day of the year in the Discordian calendar may be taken to correspond to the same day of the year in the Gregorian calendar, and vice versa (but see "St. Tib's Day dispute", below).
There are Apostle Holydays on the 5th day of each season, named after the 5 Discordian apostles: Mungday, for Hung Mung; Mojoday, for Dr Van Van Mojo; Syaday, for Sri Syadasti; Zaraday, for Zarathud; and Maladay, for Malaclypse the Elder. There are also Season Holydays on the 50th of each season: Chaoflux, Discoflux, Confuflux, Bureflux, and Afflux.
Holyday Discordian calendar Gregorian calendar
Mungday Chaos 5 January 5
Chaoflux Chaos 50 February 19
St. Tib's Day St. Tib's Day February 29
Mojoday Discord 5 March 19
Discoflux Discord 50 May 3
Syaday Confusion 5 May 31
Confuflux Confusion 50 July 15
Zaraday Bureaucracy 5 August 12
Bureflux Bureaucracy 50 September 26
Maladay The Aftermath 5 October 24
Afflux The Aftermath 50 December 8
Only these ten dates are named in the Principia Discordia - however, Discordians have felt free to invent other holidays which have become popular to varying degrees. Some of these include Discordians for Jesus/Love Your Neighbor Day (March 25/Discord 11); Jake Day (April 6/Discord 23 or occasionally May 23/Discord 70), a day to send tongue-in-cheek letters, emails or faxes to an official or bureaucracy; Towel Day (May 25/Discord 72); Mid Year's Day (July 2/ Confusion 37); and Multiversal Underwear Day (August 10/Bureaucracy 3).
Since 3171 YOLD (2005 CE), a schism has arisen within Discordianism over the St. Tib's Day rule. The Discordian Calendar, as described in the Principia Discordia, is unclear about whether the St. Tib's Day rule follows the Julian or Gregorian Calendar.
The rule as given is "occurs once every four years", which would imply that the Discordian Calendar is effectively aligned to the Julian calendar and that the year begins on December 19 Julian. In this interpretation, the Discordian and Gregorian calendars are only aligned for the two centuries between Chaos 60, 3066 YOLD (March 1, 1900 Gregorian) and Chaos 59, 3266 YOLD (February 28, 2100 Gregorian).
However, that page also claims to be a "Perpetual Date Converter from Gregorian to POEE Calendar", which implies that the Discordian and Gregorian calendars are perpetually aligned. In this interpretation, St. Tib's Day is skipped in years where the year ends in 66, unless the number of the century when divided by 4 leaves a remainder of 3 (e.g., since 31 = (7 * 4) + 3, 3166 was a leap year and 3566 will be a leap year, while 3266, 3366 and 3466 will not).