There are 52 complete weeks in a year. The year has 365 days, leaving one extra day. A leap year has 366 days, adding a second extra day. This makes 52 1/7 weeks in a normal year and 52 2/7 weeks in a leap year.
A calendar year has more than 52 weeks, but the remaining weeks are incomplete. A calendar year begins on the first day of January and ends on the last day of December. If the first day of January is a Saturday, then the next week begins on the second day of January. In such a situation, if it is a leap year, the last day of December is also a new week. Technically, this makes 54 weeks in the calendar year. Such situations are rare; the calendar year 2000 had 54 weeks.