Calendar Wiki

Year-Start Rule[]

The calendar-year starts with the Monday that starts closest to the South-Solstice. (The Winter Solstice north of the equator, the Summer-Solstice south of the equator.)

…or closest to an approximation to the South-Solstice, based on the assumption that a South-Solstice occurs exactly every 365.2422 days, starting from the actual South-Solstice of Gregorian 2017.

The arithmetical-rule that uses the approximation described in the paragraph before this one, assumes that the 2017 South-Solstice occurred exactly .6860 of the way through December 21st. ...a figure that's accurate within .0001 day.

...and is based on the December 21st, 16:27:50 time that is given for the 2017 South-Solstice.

The South-Solstice WeekDate calendar uses the arithmetical year-start rule described immediately-above.

The calendar-year starts with the Monday that starts closest to the approximate South-Solstice defined directly above.

(Our current Gregorian leapyear-rule, too, is based on an approximation—an approximation to the March Equinox, for the purpose of (relatively) consistently placing Easter with respect to that equinox.  Such an approximation is more convenient than using each year’s actual observed South-Solstice or an orbital calculation of it.)

Of course this South-Solstice Nearest-Monday year-start rule results in every year starting on a Monday, meaning that this is a fixed calendar, a calendar that starts every calendar-year on the same day of the week.

Therefore every year’s calendar is identical to every other year’s calendar, except that, while most years have 52 weeks, occasionally the South-Solstice Nearest-Monday year-start rule will result in a 53-week year. The extra week is added to the end of the year.

In the case of this South-Solstice WeekDate Calendar, that means that the leapyear will have a week called “week 53”, in addition to the usual 52 weeks.

That completes the specification and description of the South-Solstice Nearest-Monday year-start rule, used by both of my calendar-proposals, including the South-Solstice WeekDate Calendar described in this post.


Weeks are simply numbered. Dates are specified by the week-number and the day-of-the-week. Hence the calendar is the same as ISO week date, except for year-start rule.

For example, today is Gregorian January 19th.  In South-Solstice WeekDate, the date is:


That’s the 6th day (Saturday) of the 4th week of 2019. But, for informal non-international use, I prefer this format:

4 Sat

( indicate Saturday of the 4th week)

The week number 4 is one greater than the 3 that would arise in the ISO week, because the differing year-start rule causes this year (2019) to begin a week earlier the corresponding ISO week year.


  1. Minimal-ness means un-arbitrariness and naturalness.
  2. Simplicity and convenience.  Months complicate a calendar.  Durations within a year are particularly easy to determine when there are no months.
  3. Payment-periods?  Payment-periods are the main practical use of months. But payment-periods have an obvious and natural solution in a WeekDate calendar such as South-Solstice WeekDate:

An individual, a business, or a govt agency could choose to make or ask for payments on the first day of, the Monday of, every week whose week-number is divisible by 4.

So payments could be made at the Monday of Week 4, Week 8, Week 12 etc.  Likewise the Monday of Week 28 would be a payment-day, because 28 is divisible by 4. 

Obviously the Monday of the last week of the common-year, Week 52, would be a payment-day too.

Alternatively, an individual, a business, or a govt agency could instead choose to make or ask for payments on the Monday of Week 1, and every week whose week-number is 1 more than a multiple of 4.  So then, the payment-days would be Monday of Week 1, Week 5, Week 9…and Week 29, for example.

The point here is that payment-periods wouldn’t be a problem with South-Solstice WeekDate, and could easily be perfectly uniform in common-years. Of course the occasional 53 week year would introduce a nonuniformity at the end of the year, but with the Roman-Gregorian Calendar we’re used to non-uniformity.

I discuss a comparison of South-Solstice WeekDate vs South-Solstice Equal 28-Day Months at this blog-post.


Someone could object that, after more than 2000 years of using the Roman months, we’re familiar with the seasonal meanings of those months, and that the week-numbers of South-Solstice WeekDate wouldn’t have that seasonal-meaning familiarity.

Answer: With any new calendar, even one with a new year-start and year-division structure, it's possible to name dates, in that new calendar, that correspond to the Roman-Gregorian dates that bound some perceived season.

Just to give an example, it's often said that Winter north of the equator, and Summer south of the equator, correspond pretty much to December, January and February.

Well, December starts about 3 weeks before the South-Solstice. ...and December, January and February have about 13 weeks. That means that, in the South-Solstice WeekDate calendar, northern Winter and southern Summer run from week 50 through week 10 (...of the following year).

In fact, that determination is easier with WeekDate, because, like any duration, that interval's determination would be complicated by months. As I said, months are a complication.

Additionally, because the calendar-year starts very close to the South-Solstice, the week-number gives a good indication of the solar ecliptic-longitude, by which the solar-declination can be calculated or estimated.

Today, Gregorian January 19th,  has the South-Solstice WeekDate date of 2019-W04-6. That’s in the range of Week 50 thru Week 10, so that’s in northern Winter.  We’re late in Week 4, meaning it’s nearly 4 weeks since the South-Solstice. Since there are about 13 weeks of Winter, and we’re now almost 3 + 4 = 7 weeks into Winter, then we’re roughly halfway through Winter.

Thus, South-Solstice WeekDate gives you that progress-through-Winter percentage in a direct way.

That percentage is particularly convenient to determine with a WeekDate calendar, because of the simplicity of having weeks as the only year-division.

And if you reside in the Tropics or the Arctic, there likewise are a few week-numbers that bound the familiar seasons where you reside.

For another thing, the week-numbers give you information that our Roman months don’t give:  Solar ecliptic longitude  (…where the Sun is, with respect to solstices and equinoxes). That’s because Week 1 starts very close to the South-Solstice.

Of course, with engagement-calendars, with South-Solstice WeekDate, the beginnings of consensus-seasons could be marked on the appropriate dates’ day-squares.  That’s what current printed calendars now purport to do, when they tell us when their 4 seasons supposedly begin (…but announce the beginnings of astronomical quarters that they mistakenly name as terrestrial seasons).