In the Double-Month Calendar or, more precisely, Double-Month Week Calendar (DMWC), the year is divided into 6 double-months, each of which contains a whole number of weeks in a pattern of 9:9:8, i.e. there are either 56 or 63 days in one double-month.
There are several synonyms available for the double-month: sester, sextal and bimester or bim for short. The former two terms allude to a sixth of a year, while the latter is just a Latinate term for "double month" or "two months" and would be preferred if the optional subdivisions are used. These terms are used interchangeably in this article. Accordingly, this calendar may be called Sester (Week) Calendar, Sextal (Week) Calendar, Bimonth (Week) Calendar or Bim (Week) Calendar as well.
Notation[]
The ISO-like date format is ±YYYY-S-W-D. The calendar is compatible with that standard as far as possible, e.g. weeks begin on Monday.
For example, today is 2026-1-2-5, the same as 2026-W02-5 or, less obviously, 2026-01-09.
If disambiguation with similar schemes is required, e.g. from Octalendar’s ±YYYY-M-W-D with 6–7-week months, the bim number may be prefixed by a single-letter marker B for bimonth, e.g. 2026-B1-2-5.
The alternative S for sester, 'sextal or sixth was avoided because it may more readily be associated with a quarter-year season, as in the International Calendar, or a 6-month semester or also half-year semi-annum.
The hyphen separators are optional then, 2026B125. With just three digits for the subdivision of the year, this notation is more efficient than most commonly used ones that employ at least 4 digits.
Leap week[]
The leap week is appended to the last sextal at the end of the year; that means only the third sextal has just 8 weeks then, all the others have 9 weeks. This avoids a 10-week sextal and is the reason for choosing the pattern 9:9:8 over 9:8:9 or 8:9:9.
For compatibility, this happens in exactly the same years that have a W53 by ISO rules. Unfortunately, this is a very complex and seemingly irregular pattern with no simple algorithm.
Names[]
There is no agreed system for naming the 8/9-week periods, but, intuitively, most people try to fuse the respective Roman month names into portmanteux, e.g. in English:
- January + February = Jabuary, Jebuary; Fanuary, Fenuary; Uary
- March + April = Marril, Maril; Aprich, Arch; Ar
- May + June = Maine, Mayne, Mane; Jay, Junay;
- July + August = Jaust, Just, Jagust, Julust; Auly, Augly;
- September + October = Setober, Sectober, Septober; Octember, Optember; Toeber
- November + December = Nocember, Nember, Necember, Nevember; Devember, Dovember, Dember; Ember
Subdivision variants[]
Each sextal can be matched by two months of 4 or 5 weeks exactly. The possible patterns for half a year, i.e. three consecutive sextals, are
- 5:4 : 5:4 : 4:4,
- 5:4 : 4:5 : 4:4,
- 4:5 : 5:4 : 4:4 and
- 4:5 : 4:5 : 4:4.
The last one could be dubbed “4:5*” and is harmonic for the second half of the leap year. The second one, on the other hand, is equivalent to the 5:4:4 weeks per month pattern, ensuring that there are never 3 consecutive short months with just 4 weeks each. Both of these preferable patterns align well with 13-week quarters.
The “symmetric” week-month pattern 4:5:4 would yield a different pattern for sextals, 9:8:9, and 4:4:5 would result in another, 8:9:9. They are not preferred, because the leap week at the end of the year would make the sixth bim 10 weeks long.
| Week | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 13mon | Janon | Febron | Marchon | Apron | Mayon | Junion | Extron | Julon | Augon | Septon | Octon | Novon | Decon † | ||||||||||||||||||||||||||||||||||||||||
| 5:4:4 | Janaire | Febraire | Marchose | Aprilose | Mayose | Junal | Julial | Augial | Septidor | Octidor | Novidor | Decaire † | |||||||||||||||||||||||||||||||||||||||||
| 4:5:4 | Janose | Febrose | Aprial | Mayal | Julidor | Augidor | Octaire | Novaire | |||||||||||||||||||||||||||||||||||||||||||||
| 4:4:5 | Febral | Marchal | Maydor | Junidor | Augaire | Septaire | Novose | Decose † | |||||||||||||||||||||||||||||||||||||||||||||
| Quarter | quart 1 | quart 2 | quart 3 | quart 4 † | |||||||||||||||||||||||||||||||||||||||||||||||||
| 7:6 | Bol | Sar | Tan | Lit | Nas | Mon | Sam | Yul † | |||||||||||||||||||||||||||||||||||||||||||||
| 4:5* | Janose | Febrose | Marchose | Aprilose | Mayose | Junal | Julidor | Augidor | Septidor | Octidor | Novidor | Decaire † | |||||||||||||||||||||||||||||||||||||||||
| 9:9:8 | sester 1 | sester 2 | sester 3 | sester 4 | sester 5 | sester 6 † | |||||||||||||||||||||||||||||||||||||||||||||||
| 5:5:6:5 | Primoth | Secoth | Tertoth | Quaroth | Quintoth | Sixtoth | Septoth | Octoth | Novoth | Decoth † | |||||||||||||||||||||||||||||||||||||||||||
| 10:11 | quinter 1 | quinter 2 | quinter 3 | quinter 4 | quinter 5 † | ||||||||||||||||||||||||||||||||||||||||||||||||
| 8:7 | Monmonth | Tuesmonth | Wednesmonth | Thursmonth | Frimonth | Saturmonth | Sunmonth † | ||||||||||||||||||||||||||||||||||||||||||||||
| Terms | Spring semester | Summer break | Fall semester | ||||||||||||||||||||||||||||||||||||||||||||||||||
| Winter semester | Spring break | Summer semester | Fall break | Winter semester | |||||||||||||||||||||||||||||||||||||||||||||||||
| Epiphany/Hilary trimester | Trinity trimester | Michaelmas trimester | |||||||||||||||||||||||||||||||||||||||||||||||||||
See also[]
- Newcal and the 6*6*10 regular calendar use a 61-day double-month, paired with 5-, 6- or 10-day weeks and one day outside the week.