Some companies use standard retail calendars, which have twelve periods per year, but in most years each quarter has thirteen weeks—most commonly a 4-week period, then a 5-week period, then another 4-week period. Every week starts on a Sunday. Standard retail calendars have a total of exactly 52 weeks (364 days), except that once every five or six years, they have 53 weeks (371 days) to compensate for the actual number of days in a Gregorian calendar year—365 or, for leap years, 366.
Other companies use non-standard retail calendars, which can have more than or fewer than twelve periods per year, and where each individual period, quarter, half-year, and year can vary in length.
You can change the periods in fiscal years in PPM so that they reflect standard or non-standard retail calendars instead of the Gregorian calendar.