# Definition of a week

The definition of a week is specified by the International Organization for Standardization (ISO) in ** ISO 8601 standard ** (https: //www.iso. org / fr / standard / 70907.html).

According to the specifications of this standard, a week has ** 7 consecutive days ** , ** starting on Monday at 00:00:00 ** and ending ** on Sunday at 11 pm : 59: 59 ** .

A ** year ** has ** 52 weeks ** (sometimes 53), numbered ** 01 to 52 ** . Week 53 is also called a leap week.

# Definition of the first week of the year

Following the recommendations of ISO 8601, there are several equivalent definitions. One of its definitions is to consider as the first week of the year, the ** first week with at least 4 consecutive days in the same calendar year ** .

** Example ** : 2020-S01

The first week of the year 2020 comprising 4 consecutive days is the one starting on Monday 12/30/2019 and ending on Sunday 01/05/2020.

Other equivalent definition:

The ** first week ** is the ** week with the first Thursday of the calendar year ** .

# Definition of the last week of the year

Following the recommendations of ISO 8601, there are several equivalent definitions. One of its definitions is to consider as the last week of the year, the ** last week with at least 4 consecutive days in the same calendar year ** .

** Example ** : 2019-S52

The last week of the year 2019 comprising 4 consecutive days is the one starting on Monday 23/12/2019 and ending on Sunday 29/01/2020.

Other equivalent definition:

The ** last week ** is the ** week with the last Thursday of the calendar year ** .

# Example of weekly comparison

For ** weekly comparisons between several years ** , you should compare the ** same week IDs ** of the years you are comparing, independently calendar days.

Thus the week 2020-S01 is compared, for the year 2019, with the week 2019-S01, 2020-S02 is compared with the week 2019-S02, etc ...

** Example ** : comparison of weeks 08 to 12 of the years 2018 to 2020

# Example of comparing days in a week

For ** day comparisons within a week between several years ** , it is advisable to reason, not in calendar day, but ** number day of the week ** . Indeed, Monday March 16, 2020 of week 12 2020, does not compare with March 16, 2019 (which is a Saturday and belongs to week 11 2020), but with the same day of the week of the same number of week of the previous year, namely Monday March 18, 2019 from week 12 2019.

** Example ** : comparison of Mondays in weeks 11 to 12 of years 2019 to 2020

For week 12 of years 2019 and 2020, we want to compare between the 2 years on the first day of the week:

Monday March 16, 2020 for week 12 2020

Monday March 18, 2019 for week 12 2019

** Tabular illustration ** : * Mon pour Monday *

** Graphic illustration ** :