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The “Thursday rule” from ISO 8601 specifies that the first and last week of a calendar year are part of the “week year” that most of their days belong to. This happens to be equivalent to the year a 7-day week’s 4th day falls into, which is Thursday according to international standards, because Monday is the first day of the week there.

Year boundary[]

All month dates can fall in one of two weeks, except for 29 December through 2 January which can be in W52, W53 or W01.

Mapping of week dates to month dates
DL Jan01 Jan02 Jan03 Jan04 Dec27 Dec28 Dec29 Dec30 Dec31
G W01 W52 W01 F/FE
GF W52 W01 E
F W01 E/ED
FE W52 W01 D
E W01 D/DC
ED W52 W53 C
D W01 C/CB
DC W53 B*
C W53 W01 W52 B/BA
CB W52 A
B* W53 W01 A/AG
B W52
BA W52 G
A W52 W01 G/GF
AG W52 W01 F

Adaptation[]

If the rule was also applied to months etc., irregular patterns ensue. The tables below show leap year numbers in right-hand column if they differ from common years. They also list the Dominical letter which is not used by ISO but in calculations of the date of Easter.

Weeks per month depending on the weekday of 1 January
DL G GF F FE E ED D DC C CB B BA A AG
1 Jan: Mon Tue Wed Thu Fri Sat Sun
01 4 5 5 5 4 4 4
02 4 5 4 4 4 4 4 4
03 5 4 4 4 4 4 5 5 5
04 4 4 4 5 5 5 4 4 4
05 5 5 5 4 4 4 4 4 5
06 4 4 4 4 4 5 5 5 4
07 4 4 5 5 5 5 4 4 4
08 5 5 4 4 4 4 4 5 5
09 4 4 4 4 5 5 5 4 4
10 4 5 5 5 5 4 4 4 4
11 5 4 4 4 4 4 4 5 5
12 4 4 4 5 5 5 5 4 4
Weeks per sixth-year / bimester depending on the weekday of 1 January
DL G GF F FE E ED D DC C CB B BA A AG
1 Jan: Mon Tue Wed Thu Fri Sat Sun
B1 8 9 9 9 9 8 8 8
B2 9 8 8 8 9 9 9 9 9
B3 9 9 9 8 8 8 9 9 9
B4 9 9 9 9 9 8 8 9 9
B5 8 9 9 9 9 9 9 8 8
B6 9 8 8 8 9 9 9 9 9
Weeks per quarter-year / trimester depending on the weekday of 1 January
DL G GF F FE E ED D DC C CB B BA A AG
1 Jan: Mon Tue Wed Thu Fri Sat Sun
Q1 13 13 13 13 12 13 13 13
Q2 13 13 13 13 13 13 13
Q3 13 13 13 13 14 14 13 13 13
Q4 13 13 13 14 14 13 13 13
Weeks per third-year / quadmester depending on the weekday of 1 January
DL G GF F FE E ED D DC C CB B BA A AG
1 Jan: Mon Tue Wed Thu Fri Sat Sun
T1 17 17 17 18 18 17 17 17
T2 18 18 18 17 17 17 17 18 18
T3 17 17 17 18 18 18 18 17 17
Weeks per half-year / semester depending on the weekday of 1 January
DL G GF F FE E ED D DC C CB B BA A AG
1 Jan: Mon Tue Wed Thu Fri Sat Sun
H1 26 26 26 26 25 26 26 26
H2 26 26 26 27 27 27 26 26 26
Weeks per year depending on the weekday of 1 January
DL G GF F FE E ED D DC C CB B BA A AG
1 Jan: Mon Tue Wed Thu Fri Sat Sun
Year 52 52 52 53 53 52 52 52
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