There are fifty-two (52) complete weeks in a (Julian) year plus an extra day, or two in a leap year. So a year will contain 52 1/7 or 52 2/7 weeks.
Arrangement of the days on a calendar: The days on the calendar will be arranged in 53 weeks, at least one of which will have less than 7 days. In the rarest of cases, the calendar can overlap 54 weeks in one year : January 1 and December 31 in their own separate weeks.
Perhaps even more fundamentally there appear to be about 52.18 weeks (365 days, 5 hours, 48 minutes 45.19 seconds) in a mean solar or tropical year. However, note that this value is changing based on the changes to the Earth's path around the sun. The Gregorian year, which is the internationally accepted civil year based on the Julian year, averages only 52.1775 solar weeks.