Thus, if a Gregorian year contains 52 Thursdays then the corresponding ISO calendar year has 52 weeks and if the Gregorian year containsĥ3 Thursdays then the corresponding ISO calendar year has 53 weeks.

Examples of these extreme cases in the recent past are December 2014Īs Thursday is the middle day of the ISO week, each Thursday in a Gregorian calendar year belongs to the same ISO calendar year. Likewise, the last week of the ISO calendar year can end as early as 28 December and as late as 3 January Likewise, the last week of the ISO calendar year is the last week that contains at least four days of the month of December.Īccording to this rule the first week of the ISO calendar year can begin as late as 4 January and as early as 29 December The numbering of the ISO calendar weeks is defined (sect. 2.2.10 on p. 5) as: the first calendar week of a year is that one which includes the first Thursday of that yearĪnd the last calendar week of a calendar year is the week immediately preceding the first calendar week of the next calendarĪnother way of stating the above rule is that the first week of the ISO calendar year is the earliest week that contains at least four days The week in the ISO calendar is defined (sect. 2.2.8 on p. 5) as a: time interval of seven calendar days starting with a Monday identified by its ordinal number On average, however, it remains in step with the Gregorian calendar. The ISO calendar corresponds with the Gregorian calendar (sect. 3.2.1 on pp. 7-8) and uses the same year number but as its length isĭefined to be an integral number of weeks, its beginning can deviate up to a few days from 1 January and its end likewise can deviate up toĪ few days from 31 December. The calendar defined in the ISO standards 20 is commonly Repealed on 1 June 1988 with the imminent introduction of ISO 8601. This week numbering scheme was introduced earlier by the ISO with the standard ISO 2015:1976 ( Numbering of Weeks) and was Mathematics of the ISO 8601 Calendar - The ISO Calendar The ISO 8601 Calendar

Week number calendar