Thursday rule

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	W01					W52			W01		E
F	W01					W52			W01		E/ED
FE	W01					W52		W01			D
E	W01					W52		W01			D/DC
ED	W01					W52	W53				C
D	W01					W52	W53				C/CB
DC	W01					W53					B*
C	W53			W01		W52					B/BA
CB	W53			W01		W52					A
B*	W53		W01								A/AG
B	W52										A/AG
BA	W52					W52					G
A	W52	W01				W52					G/GF
AG	W52	W01				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	E	ED	D	C	CB	B	BA	A
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	F	E	ED	D	DC	C	CB	B	A
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	F	E	ED	D	C	B	BA	A
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	F	E	ED	D	C	CB	B	A
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