Weekly timescales represent the 52 weeks (4-4-5 or 13 4-weeks) or 364 days in a year (52 *7 =364). This is one short of the 365 days in a calendar year. Consequently, the financial year end occurs earlier in subsequent years until it is possible to fit in an extra 53rd week (approximately every 6 years). Leap years complicate the picture by providing an extra day every fourth year. To define when the 53-week year occurs, it is sufficient to set the day of the week on which the financial year ends and the period in which you would like to add the 53rd week. Anaplan calculates the years in which the 53rd week is due.
In Model Settings > Time > Model Calendar, select a day for the end of the fiscal year (for example, Last Saturday in December) and the period in which the 53rd week is to fall (for example, Period 12 in a 13 4-week timescale). This is sufficient to define which years contain 53 weeks.