The number of each day inside a year is provided from 1567.
The standard numbers of the weeks start in 1976.
Recommandations International Organisation of Standards :
- Monday is considered as the first day of the week.
- The weeks of a year are numbered 01 to 52 (sometimes 53).
- The week numbered 01 is the week including the first Thurday of January.
- It happens that the week 53 exists: common year ending on a Thursday or leap year ending on a Thursday or a Friday).