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The ISO 8601 definition for week 01 is the week with the first Thursday of the Gregorian year (i.e., of January) in it.The following definitions based on properties of this week are mutually equivalent, since the ISO week starts with Monday:
* It is the first week with a majority (4 or more) of its days in January.
* Its first day is the Monday nearest to 1 January.
* It has 4 January in it. Hence the earliest possible first week extends from Monday 29 December (previous Gregorian year) to Sunday

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