Editing Week (section)

==Numbering==
<!-- This section is linked from [[Week number]] -->
{{Further|Leap week calendar}}

Weeks in a Gregorian calendar year can be numbered for each year. This style of numbering is often used in European and Asian countries. It is less common in the U.S. and elsewhere.

===The ISO week date system===
{{More citations needed section|date=June 2020}}

The system for numbering weeks is the [[ISO week date]] system, which is included in [[ISO 8601]]. This system dictates that each week begins on a Monday and is associated with the year that contains that week's Thursday.

====Determining ''Week 1''====
In practice week 1 (''W01'' in ISO notation) of any year can be determined as follows:

* If 1 January falls on a Monday, Tuesday, Wednesday or Thursday, then the week of 1 January is Week 1. Except in the case of 1 January falling on a Monday, this Week 1 includes the last day(s) of the ''previous'' year.
* If 1 January falls on a Friday, Saturday, or Sunday, then 1 January is considered to be part of the last week of the ''previous'' year. Week 1 will begin on the first Monday after 1 January.

Examples:

* Week 1 of 2015 (''2015W01'' in ISO notation) started on Monday, 29 December 2014 and ended on Sunday, 4 January 2015, because 1 January 2015 fell on Thursday.
* Week 1 of 2021 (''2021W01'' in ISO notation) started on Monday, 4 January 2021 and ended on Sunday, 10 January 2021, because 1 January 2021 fell on Friday.

====Week 52 and 53====
It is also possible to determine if the last week of the previous year was Week 52 or Week 53 as follows:

* If 1 January falls on a Friday, then it is part of Week 53 of the previous year (W53-5). 
* If 1 January falls on a Saturday,
** then it is part of Week 53 of the previous year if that is a [[leap year]] (W53-6),
** and part of Week 52 otherwise (W52-6), i.e. if the previous year is a common year. 
* If 1 January falls on a Sunday, then it is part of Week 52 of the previous year (W52-7).

====Schematic representation of [[ISO week date]]====
{| class="wikitable"
|+ [[Dominical letter]](s) plus weekdays, dates and week numbers at the beginning and end of a year
|- valign=bottom
! rowspan=2 | Dominical<br>letter(s)<sup>1</sup>
! colspan=7 | Days at the start of January
! colspan=4 | Effect<sup>1,2</sup>
! colspan=7 | Days at the end of December<sup>1</sup>
|- valign=bottom
! 1<br>Mon || 2<br>Tue || 3<br>Wed ||4<br>Thu || 5<br>Fri || 6<br>Sat || 7<br>Sun
! {{nowrap|W01-1<sup>3</sup>}} !! 01 Jan week !! ... !! 31 Dec week
! 1<br>Mon<sup>4</sup> || 2<br>Tue || 3<br> Wed || 4<br>Thu || 5<br>Fri || 6<br>Sat || 7<br>Sun
|- align=center
! G(F)
| 01 || 02 || 03 || 04 || 05 || 06 || 07
| 01 Jan || W01 || ... || <u>W01</u>
| {{nowrap|31 (30)}} || align=right | (31) ||  ||  ||  ||  ||
|- align=center
! F(E)
|  || 01 || 02 || 03 || 04 || 05 || 06
| <u>31 Dec</u> || W01 || ... || <u>W01</u>
| 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||  ||  ||  ||
|- align=center
! E(D)
|  ||  || 01 || 02 || 03 || 04 || 05
| <u>30 Dec</u> || W01 || ... || title="W53 when ending year is leap, W01 otherwise." | <u>W01</u> (W53)
| 29 (28) || 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||  ||  ||
|- align=center
! D(C)
|  ||  ||  || 01 || 02 || 03 || 04
| <u>29 Dec</u> || W01 || ... || W53
| 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||  ||
|- align=center
! C(B)
|  ||  ||  ||  || 01 || 02 || 03
| 04 Jan || <u>W53</u> || ... || W52
| 27 (26) || 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||
|- align=center
! B(A)
|  ||  ||  ||  ||  || 01 || 02
| 03 Jan || title="W53 when ending year is leap, W52 otherwise." | <u>W52</u> (<u>W53</u>)|| ... || W52
| 26 (25) || 27 (26) || 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} || style="width:7ex" align=right | (31)
|- align=center
! A(G)
|  || || || || || || 01
| 02 Jan || <u>W52</u> || ... || title="31 December is in W01 only if it is in a leap year."| W52 (<u>W01</u>)
| 25 (31) || 26 (25) || 27 (26) || 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} 
|}

'''Notes'''<br />
1. Numbers and letters in parentheses, ( ), apply to March − December in leap years.<br />
2. <u>Underlined</u> numbers and letters belong to previous year or next year.<br />
3. First date of the '''first''' week in the year.<br />
4. First date of the '''last''' week in the year.

===Other week numbering systems===
In some countries, though, the numbering system is different from the ISO standard. At least six numberings are in use:<ref>{{cite web |url=http://www.pjh2.de/datetime/weeknumber/wnd.php?l=en |title=Weeknumber sorted by definition |author=Peter Johann Haas |date=26 January 2002 |website=pjh2.de |access-date=3 April 2015 |archive-date=9 February 2016 |archive-url=https://web.archive.org/web/20160209193500/http://www.pjh2.de/datetime/weeknumber/wnd.php?l=en |url-status=dead }}</ref><ref>{{cite web |url=http://www.merlyn.demon.co.uk/weekinfo.htm |title=Calendar Weeks |publisher=J. R. Stockton |url-status=usurped |archive-url=https://web.archive.org/web/20140113000828/http://www.merlyn.demon.co.uk/weekinfo.htm |archive-date=13 January 2014}}</ref> {{Dubious|date=October 2015}}

{|class="wikitable"
! System
! First day of week
!colspan="3"| First week of year contains
!Can be last week of previous year
! Used by or in
|-
| ISO 8601 || [[Monday]] || {{nowrap|4 January}} || {{nowrap|1st Thursday}} || {{nowrap|4–7 days of year}} || align=center | yes || EU (exc. Portugal) and most of other European countries, most of Asia and Oceania
|- 
| Middle Eastern || [[Saturday]] || {{nowrap|1 January}} || {{nowrap|1st Friday}} || {{nowrap|1–7 days of year}} || align=center | yes || Much of the Middle East
|-
| Western traditional || [[Sunday]] || {{nowrap|1 January}} || {{nowrap|1st Saturday}} || {{nowrap|1–7 days of year}} || align=center | yes || Canada, United States, Iceland, Portugal, Japan, Taiwan, Thailand, Hong Kong, Macau, Israel, Egypt, South Africa, the Philippines, and most of Latin America
|-
| [[Broadcast Calendar]] || [[Monday]] || {{nowrap|1 January}} || {{nowrap|1st Sunday}} || {{nowrap|1–7 days of year}} || align=center | yes || Broadcast services in the United States<ref>{{Cite web|title=Broadcast Calendars {{!}} RAB.com|url=http://www.rab.com/public/reports/broadcastCalendar.cfm?type=nm|access-date=26 May 2021|website=www.rab.com}}</ref>
|}

Because the week starts on either Saturday, Sunday, or Monday in all these systems, the days in a [[workweek]], Monday through Friday, will always have the same week number within a calendar week system. Quite often, these systems will agree on the week number for each day in a workweek:

* In years where 1 January is a [[Common year starting on Monday|Monday]], [[Common year starting on Tuesday|Tuesday]], [[Common year starting on Wednesday|Wednesday]], or [[Common year starting on Thursday|Thursday]], all of the above week numbering systems will agree. 
* [[Common year starting on Friday|In years where 1 January is a Friday]], ISO-8601 will be different, but the rest will agree. 
* [[Common year starting on Saturday|In years where 1 January is a Saturday]], ISO-8601 and the Middle Eastern system will agree, being different from Western Traditional and the Broadcast Calendar which will agree. 
* [[Common year starting on Sunday|In years where 1 January is a Sunday]], the Broadcast Calendar will be different, but the rest will agree.

Note that this agreement occurs only for the week number of each day in a work week, not for the day number within the week, nor the week number of the weekends.

{{anchor|Epidemiology}}The '''epi week''' ('''epidemiological week''') is used to report healthcare statistics, as with COVID-19 cases:<ref>{{cite journal |title=Norms and Standards in Epidemiology: Epidemiological Calendar 2000 |url=https://www3.paho.org/english/sha/be993calend.htm |issn=0256-1859 |journal=Epidemiological Bulletin |volume=20 |number=3 |date=September 1999 |access-date=2024-09-11 |publisher=[[Pan American Health Organization]]}}</ref>
<blockquote>The epidemiological week begins on Sunday and ends on Saturday. The first epidemiological week of the year ends on the first Saturday of January, provided that it falls at least four or more days into the month. Therefore, the first epidemiological week may actually begin in December of the previous year.</blockquote>

====Uses====
The [[semiconductor package#Date code|semiconductor package date code]] is often a 4 digit date code YYWW where the first two digits YY are the last 2 digits of the calendar year and the last two digits WW are the two-digit week number.<ref>{{cite web |publisher=Texas Instrument |url=http://focus.ti.com/quality/docs/gencontent.tsp?templateId=5909&navigationId=12626&contentId=153966 |title=Quality & Lead-free (Pb-free): Marking Convention |url-status=dead |archive-url=https://web.archive.org/web/20140405090712/http://focus.ti.com/quality/docs/gencontent.tsp?templateId=5909&navigationId=12626&contentId=153966 |archive-date=5 April 2014}}</ref><ref>{{cite web |publisher=Fairchild Semiconductor |url=http://www.fairchildsemi.com/support/packaging/topmark/dateCode4/ |title=Top Mark Convention – 4-Digit Date Code |url-status=dead |archive-url=https://web.archive.org/web/20140714111530/http://www.fairchildsemi.com/support/packaging/topmark/dateCode4/ |archive-date=14 July 2014}}</ref>

The [[tire code#DOT code|tire date code mandated by the US DOT]] is a 4 digit date code WWYY with two digits of the week number WW followed by the last two digits of the calendar year YY.<ref>{{cite web |url=https://www.law.cornell.edu/cfr/text/49/574.5 |title=49 CFR 574.5 – Tire identification requirements |website=Legal Information Institute}}</ref>