Monday—Sunday: the ISO week system
In the ISO 8601 week-date system, the week starts on Monday and ends on Sunday. The first week of the year is defined as the week containing 4 January. Equivalently, it is the week containing the year's first Thursday. This means that 1 January does not necessarily belong to Week 1. It can sometimes fall in Week 52 or Week 53 of the previous week-year. For example, if 1 January falls on a Friday, the Monday—Sunday week containing it began in December of the previous calendar year. Under ISO week numbering, that week belongs to the previous week-year.Sunday—Saturday: the traditional Sunday-start system
In a Sunday-start system, the week runs from Sunday to Saturday. A common convention is to define Week 1 as the week containing 1 January. For example, if 1 January is a Wednesday, Week 1 runs from the preceding Sunday through the following Saturday, and therefore begins in December of the previous calendar year. This is one reason why a date can have a different week number in a Sunday-start calendar from the number it has under ISO 8601.Saturday—Friday: a Saturday-start system
A Saturday-start week runs from Saturday to Friday. If Week 1 is defined as the week containing 1 January, there is an equivalent way of expressing the same rule: Week 1 is the week containing the year's first Friday. These two definitions are equivalent.The underlying pattern
The interesting point is that the start day of the week and the rule for determining Week 1 are separate concepts.Why this matters
As a result, the same calendar date can have different week numbers depending on the week-numbering convention being used.We support three week-date systems for displaying week numbers
1. Monday—Sunday (ISO 8601): the week containing 4 January
is designated as Week 1. When a week
runs from Monday to Sunday, the first week
is the week containing 4 January, which
is also the week containing the first Thursday of the year.
2. Sunday—Saturday: the week containing 1 January
is designated as Week 1. When a week
runs from Sunday to Saturday, the first week
is the week containing 1 January, which
is also the week containing the first Saturday of the year.
3. Saturday—Friday: the week containing 1 January
is designated as Week 1. When a week
runs from Saturday to Friday, the first week
is the week containing 1 January, which
is also the week containing the first Friday of the year.