Most of the world numbers weeks according to the ISO 8601 standard. A week starts on Monday, and week 1 is the week that contains the first Thursday of the year, which is always the week that contains 4 January.
Week 1 of 1991 starts on December 31, 1990 and ends on January 6, 1991 (ISO 8601, Monday to Sunday). 1991 has 52 ISO weeks.
Because of this rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year.
In the United States, weeks usually start on Sunday and week 1 is the week that contains 1 January, so the US week number can differ from the ISO week number.
Frequently Asked Questions (FAQ)
How many weeks does 1991 have?
1991 has 52 ISO weeks.
When does week 1 of 1991 start?
Week 1 of 1991 starts on December 31, 1990 and ends on January 6, 1991 (ISO 8601, Monday to Sunday).
When does the last week of 1991 end?
Week 52 of 1991 starts on December 23, 1991 and ends on December 29, 1991 (ISO 8601, Monday to Sunday).
Which years have 53 weeks?
The next years with 53 ISO weeks are 1992, 1998, 2004, 2009, 2015.
Why do the US and Europe show different week numbers?
Europe follows ISO 8601: weeks start on Monday and week 1 is the week with the first Thursday of the year. In the US, weeks usually start on Sunday and week 1 is the week that contains 1 January, so the two numbers can differ by one.