In the Double-Month Calendar, the year is divided into 6 sesters (alias sextals or bimesters) or double-months, each of which contains a whole number of weeks in a pattern of 9:9:8. The leap week is appended to the last sextal, that means only the third sextal has just 8 weeks then.
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.
Each sextal can be matched by two months of 4 or 5 weeks exactly. The possible patterns for 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, “4:5*”, is harmonic for the second half of the leap year, but the second one is equivalent to the 5:4:4 pattern, ensuring that there are never 3 consecutive months with just 4 weeks each. Both align well with 13-week quarters.
| 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 | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5:5:6:5 | month 1 | month 2 | month 3 | month 4 | month 5 | month 6 | month 7 | month 8 | month 9 | month 10† | |||||||||||||||||||||||||||||||||||||||||||||||||||
| 10:11 | quinter 1 | quinter 2 | quinter 3 | quinter 4 | quinter 5† | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 9:9:8 | sester 1 | sester 2 | sester 3 | sester 4 | sester 5 | sester 6† | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 8:7 | Monmonth | Tuesmonth | Wednesmonth | Thursmonth | Frimonth | Saturmonth | Sunmonth† | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 7:6 | month 1 | month 2 | month 3 | month 4 | month 5 | month 6 | month 7 | month 8† | |||||||||||||||||||||||||||||||||||||||||||||||||||||
| 13 | quarter 1 | quarter 2 | quarter 3 | quarter 4† | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 4:5* | January | February | March | April | May | June | July | August | September | October | November | December† | |||||||||||||||||||||||||||||||||||||||||||||||||
| 5:4:4 | January | February | March | April | May | June | July | August | September | October | November | December† | |||||||||||||||||||||||||||||||||||||||||||||||||
| 4:5:4 | January | February | March | April | May | June | July | August | September | October | November | December† | |||||||||||||||||||||||||||||||||||||||||||||||||
| 4:4:5 | January | February | March | April | May | June | July | August | September | October | November | December† | |||||||||||||||||||||||||||||||||||||||||||||||||
| 13×4 | January | February | March | April | May | June | X | July | August | September | October | November | December† | ||||||||||||||||||||||||||||||||||||||||||||||||
| Academic (example) | Spring semester | Fall semester | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Winter semester | Summer semester | Summer break | Winter semester | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Epiphany/Hilary trimester | Trinity trimester | Michaelmas trimester | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
