ISO week number for a date, or the dates for a given week.
ISO week number
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How this works
ISO weeks run Monday to Sunday. Week 1 is the week containing the year's first Thursday —
equivalently, the week containing 4 January.
Because week 1 is defined by the first Thursday, the last few days of December can fall in week 1 of the following year, and the first few days of January can fall in the last week of the previous year.
Most years have 52 ISO weeks; years where 1 January falls on a Thursday (or it is a leap year starting on a Wednesday) have 53.
This differs from the "week of the year" some US calendars show, which typically starts weeks on Sunday and does not follow the ISO rule.
Why does 1 January sometimes show as week 52 or 53?
ISO weeks belong to the year that contains the Thursday of that week. If 1 January falls on a Friday, Saturday or Sunday, that week's Thursday is still in the previous year, so the date is reported as being in the last week of the previous year.
What day does an ISO week start on?
Monday. This is different from some calendar conventions (common in the US) that start the week on Sunday.
Can a year have 53 weeks?
Yes — any year where 1 January falls on a Thursday, or a leap year where 1 January falls on a Wednesday, has 53 ISO weeks instead of the usual 52.