ISO Week Numbering
ISO 8601 defines week 1 as the week containing the first Thursday of the year (or equivalently, the week containing January 4th). Weeks start on Monday and end on Sunday. Some years have 53 weeks.
Frequently Asked Questions
No. ISO week 1 is the week containing the first Thursday of the year. January 1 can fall in week 52 or 53 of the previous year if it is a Friday, Saturday, or Sunday.