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The ISO week and ISO year are issued by the International Organization for Standardization and are used to create a standard and a comparable week and year number. A week number in ISO will always start on a Monday, independent of the actual first date of the year (January first). The first week of the year always contains the 4th of January & the Thursday is always a January-day. So basically the first week can be as below, or anything in between:
| Day of Week | Date |
| Day of Week | Date |
|---|
| Monday | 4th of January | Monday | 29th of December |
| Tuesday | 5th of January | Tuesday | 30th of December |
| Wednesday | 6th of January | Wednesday | 31st of December |
| Thursday | 7th of January |
Friday8th Saturday 9th Sunday10th | Day of Week | Date |
|---|
| Monday | 29th of December |
| Tuesday | 30th of December |
| Wednesday | 31st of December |
Thursday | 1st Friday2nd Saturday 3rd | 10th of January | Sunday | 4th of January |
Due to this mapping, the ISO year has 52 weeks (364 days) or 53 weeks (371 days). The week number also determines which ISO year a date has. E.g. December 31st 2019 belongs to ISO week 1 of the next year, so the ISO year of 31-12-2019 is 2020.