The ISO 8601 Standard
The International Organization for Standardization (ISO 8601) defines an unambiguous worldwide standard for calendar weeks. Under ISO 8601, Monday is always designated as Day 1 of the week, and Sunday is Day 7.
How Week 1 is Determined
Week 1 of any calendar year is the week that contains the first Thursday of that year. Equivalently, it is the week containing January 4th. This guarantees that Week 1 always contains at least four days of the new calendar year.
Years with 53 Weeks
Most years have 52 ISO weeks. However, any year starting on a Thursday, or a leap year starting on a Wednesday, will contain 53 weeks (371 days).