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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 | Thursday | 1st of January | |
Friday | 8th of January | Friday | 2nd of January | |
Saturday | 9th of January | Saturday | 3rd of January | |
Sunday | 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.