This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
International standard ISO-8601 (mentioned in section 6.6) assigns a
number to each week of the year. A week that lies partly in one year
and partly in another is assigned a number in the year in which most
of its days lie. This means that
Week 1 of any year is the week that contains 4 January,
Week 1 of any year is the week that contains the first
Thursday in January.
Most years have 52 weeks, but years that start on a Thursday and leap
years that start on a Wednesday have 53 weeks.
Note: This week numbering system is not commonly used in the United