52 full weeks fit into a year. A common year has one extra day, and a leap year has two.

1. 365 ÷ 7 = 52 remainder 1
2. 366 ÷ 7 = 52 remainder 2
3. 52 × 7 = 364 days

ISO week numbers start on Monday and assign week 1 to the week containing January 4. Some ISO week-numbering years have 53 weeks because their boundaries can differ from January 1 and December 31. Use actual dates when planning weekly payments or deadlines. A weekday can occur 53 times in a calendar year if it falls on one of the extra days.