Calendar Wiki

Many people who design reformed calendars, which they would like to see adopted worldwide soon, are lead by a primary motive: usually either improved astronomic correspondence or improved regularity. I will ignore reform proposals which are even less likely because they only make sense within a larger framework of cultural reforms.

What many wannabe reformers do not take into account enough is the humane factor. A new calendar needs to offer clear benefits and, in comparison, negligible drawbacks to make the painful transition worthwhile. The problem with the civil (Gregorian) calendar, like with so many other cultural conventions, is that it is good enough for most people and purposes despite all of its shortcomings. There is hardly any incentive for change. We are all used to deal with this shit.


It sounds nice that each month could have the same number of days or even a constant number of full weeks or at least a systematic distribution of only two different month lengths. It also seems useful if each date had the same day of the week each and every year. But people quickly realize what this means for their birthday or their wedding anniversary or for some other annually recurring date that is important to them:

  1. The combination of day and month would not exist at all in the new calendar.
  2. It would never fall on a weekend where they would prefer to celebrate it.

They usually become opposed to the idea of calendar reform immediately and lastingly.

Even if you tell people of the first kind that they can look up their birthday in the new calendar when applied to the year they were born in to find out the new date they should celebrate it on, this does not satisfy everyone, because they either may fall in the second category or they are strongly attached to their personal day-and-month date. The latter may be worse if the month of the event changes or if they will lose a simple to memorize numeric date (like -01-23 or -11-11) or if they will get a new one with unlucky numbers by superstition (e.g. --13).

To keep using the same old-style date, assumes a continuous, absolute and universal calendar that can be reformed. This was commonly done in Europe and European colonies after the Gregorian reforms had been adapted locally because neither had new dates been added nor had old ones been deleted. To use the new-style date by looking at the proleptic original date, assumes that the calendar was not just reformed but was substituted by a new one. Calendar reformers who want to recommend or enforce the latter approach are probably proposing a more radical redesign and they should develop, for instance, new month names (or at least deprecate the traditional ones) to emphasize this fact.


Let's look at one of the favorite holidays of many a people around the world: Christmas Day is now usually celebrated on 25 December, but the holiday already starts the evening before on Christmas Eve and in some places extends to the following day (Boxing Day). It is also preceded by up to 4 Sunday-through-Saturday weeks of Advent, i.e. the longest pre-Christmas period is where 25 December is a Sunday. Some sects even (used to) have a total of 6 weeks of Advent. Christmastide lasts at least until 6 January the next year or the Sunday thereafter, i.e. the feast of Epiphany, but in some denominations christmastide is hardly distinguished from Epiphany season and thus runs through 2 February, Candlemas or Feast of the Presentation, or even until and including Carnival, which is immediately followed by Lent starting on Ash Wednesday whose date depends on Easter. The Feast of the Annunciation or Lady Day is celebrated 9 months before Christmas Day, i.e. on 25 March, and thus in late Lent or early Eastertide (because the earliest date for Easter Sunday is 22 March), and it used to mark the turning of years in some places, while others had their New Year (about) a month earlier at the end of February.

25 December is the 7th-last day of the twelfth and last month of the year. It is thus the 359th day of a common year or the 360th of a leap year. It usually falls into week W52 by ISO 8601 rules, but into the weekend of W51 in common years starting (and ending) on a Friday or Saturday (Dominical letters C and B) and onto W51-7 (i.e. Sunday) in leap years starting on Friday. In years that have a 53rd week by ISO rules, 25 December will be in W52. That means it is always either in the last or the second to last week of the year.

Due to the pecularities of the Gregorian 400-year leap cycle, 25 December falls more often on a Tuesday, Friday or Sunday (58 times, 14.5%) than on a Wednesday or Thursday (57, 14.25%) while Monday and Saturday (56, 14%) are the least frequent.

The cultural assimilation of prior feasts in pre-Medieval Europe made Christmas fall intentionally close to but after the Southern solstice on 21 or 22 December which marks the begin of astronomical winter in the Northern hemisphere. This is the 4th and last astronomical season of the year, and also the beginning of the tropical zodiac sign of Capricorn. The date has been chosen quite arbitrarily and almost certainly does not match the actual birth date of Jesus of Nazareth, if he ever existed, but neither does the epoch of the calendar: the date was likely in neither year 0000 (alias 1 BC) nor 0001 (alias 1 AD), but a bit earlier. That means, for Christmas, unlike birthdays and many national holidays, we cannot calculate the respective new date for a specific day in the past.

The current offset between the Julian and Gregorian calendars is 13 days and thus several Orthodox Churches celebrate Christmas on Gregorian 7 January, i.e. Julian 25 December. Other holidays adjust accordingly.

Hanukkah (25 Kislev through 2 or 3 Tevet) partially has developed into a Christmas-like holiday (with presents etc.) and Kwanzaa (26 December through 1 January) was developed as one, all occurring around the same time of the year.

All of these relations will contribute more or less to the acceptance of the date for Christmas in a reformed calendar. Proposals cannot satisfy each and every stakeholder, but their developers must try to respect all of them. That means they need to explain their choice well.

Many calendar designs move the leap item to the end of the year and most have regular month or quarter lengths and thus the twelfth or last month often systematically begins on a fixed day of the week (dow). This may be the same as the first day of the year, but in several perpetual annual patterns it is not. The earliest possible date for Christmas Day if all Advent Sundays should be part of the same month would be the 23rd which is on the same dow as the 30th.

Day of the week for relevant days of the month
1st 25th 28th 30th 31st
Mon Thu Sun Tue Wed
Tue Fri Mon Wed Thu
Wed Sat Tue Thu Fri
Thu Sun Wed Fri Sat
Fri Mon Thu Sat Sun
Sat Tue Fri Sun Mon
Sun Wed Sat Mon Tue
Details for relevant dates by calendar proposals with end-of-year leap
Vulgar names Descriptive name 25th in last month Sunday before New Year's Eve
CCC&T 30:30:31 Sun Sun -12-25
HHPC 30:30:31 Mon Mon -12-31
30:31:30 Sun Mon -12-24
Sym010 30:31:30 Mon Tue -12-30
World 31:30:30 Sun Mon -12-24
Aristeo 31:30:30 Mon Tue -12-30
30:31 Sun Tue -12-23 (common), -12-30 (leap)
30:31 Mon Wed -12-29
30:31 Sun Tue -12-23 (common), -12-30 (leap)
30:31 Mon Wed -12-29
30x11* 30*11+x Sun Fri -12-28 (common), -12-35 (leap)
30x11* 30*11+x Mon Sat -12-34
Cotsworth, Eastman 28*13 Sun Wed -13-22
IFC 28*13 Mon Thu -13-28 (or -13-21)
28:28:35 Sun Wed -12-29
28:28:35 Mon Thu -12-28
28:35:28 Sun Wed -12-22
Sym454 28:35:28 Mon Thu -12-21
Bonavian 35:28:28 Sun Wed -12-22
35:28:28 Mon Thu -12-21