Find the ISO 8601 week number for a date — the international standard used across Europe and by most calendar and project management tools.
ISO 8601 defines week 1 as the week containing the year's first Thursday. This means the Monday of week 1 always falls in the same calendar year, or at the end of December the previous year:
This is why the first days of January can sometimes belong to the last week of the previous year, and the last days of December can belong to week 1 of the following year.
Most years have 52 weeks, but years where January 1st falls on a Thursday (or a leap year where it falls on a Wednesday) get 53 weeks according to ISO 8601.
Yes. If January 1st falls on a Friday, Saturday or Sunday, the Monday of week 1 falls at the end of December the previous year.