The South-Solstice Equal 28-Day Months by Michael Ossipoff is a proposed leap week calendar with weeks starting on Monday, arranged into 13 months of 4 weeks each, and the first week starting close to the Northern winter solstice (i.e. the Southern summer solstice).

## Year-Start Rule

This calendar can be used with either an astronomical or an arithmetical rule to determine the start of the year, which are the two variants of the “South-Solstice Nearest-Monday year-start rule”:

1. The calendar year starts with the Monday that starts nearest to the South-Solstice.
2. The date of the solstice can be approximated based on the assumption that it occurs exactly every 365.2422 days, starting from the actual South-Solstice of Gregorian 2017.
3. For the purpose of the above-stated arithmetical rule, it is assumed that the 2017 South-Solstice occurred exactly .6860 of the way through December 21st. (...a figure accurate within .0001 days, and based on the December 21, 16:27:50 UTC time that is given for that solstice.)

The arithmetical approximation is the preferred variant for South-Solstice Equal 28-Day Months.   It repeats after 35,000 years which contain exactly 1,826,211 weeks, but the assumption that it occurs every 365.2422 days, would almost certainly be modified before the 35,000 years pass, to take account of the changing length of the tropical year.

## Year-Division Structure

Month layout
Week 1 2 3 4 5 6 7
Mon Tue Wed Thu Fri Sat Sun
W1 01 02 03 04 05 06 07
W2 08 09 10 11 12 13 14
W3 15 16 17 18 19 20 21
W4 22 23 24 25 26 27 28

As in other 13×28 calendars, in a common year there are 13 equal months, each with 28 days.

Roughly every 5.6 years, the year-start rule will result in a year whose length is 53 weeks instead of 52 weeks. In such a year, referred to as a “leap year”, that extra week is added as part of the last month, which thereby becomes a 35-day month.

## Month-Naming

The names of the months are not specified, but it is suggested that they be named by numbering. In particular, Esperanto numbering is suggested for month-naming:

Unua, Dua, Tria, Kvara, Kvina, Sesa, Sepa, Oka, Nua, Deka, DekUnua, DekDua, DekTria

## Date Designation

The day of the month is designated by the week of the month (1–4, 5 in the final month of a leapyear) and the day of the week (1–7). For instance, 25 January 2019 is indicated by:

Month 2, Week 1, Friday

This “week-&-day” format is due to Karl Palmen, who uses it in his Week & Month Calendar.