Week number
Under ISO 8601 a week runs Monday to Sunday and week 1 is the one containing the first Thursday of the year. So if 1 January is a Friday, that week has only three days in the new year and belongs to the previous year as week 53. It is why the week-numbering year is shown separately — it does not always match the calendar year.
ISO 8601 weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the year. That means early January can belong to week 52 or 53 of the previous year — 1 January 2021 fell in week 53 of 2020.
How to find a week number
Two competing conventions cause most week-number confusion. ISO 8601, used across Europe and in most software, starts weeks on Monday and defines week 1 by the first Thursday. The North American convention starts weeks on Sunday and treats the week containing 1 January as week 1. They disagree by a week for a good part of the year, so a week number is only meaningful alongside the convention it uses.
Questions
Leave the date at today and read the ISO week.