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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.

Relation to other leap week calendars
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
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