Week numbers are reported using the ISO 8601 week date system where the number of weeks in one year can be either 52 or 53. Week 01 is the week with the Gregorian year’s first Thursday in it. For example, if January 3 is a Thursday, Week 01 begins on Dec 31, 30, or 29 of the previous year depending on whether Monday, Sunday, or Saturday respectively is chosen as the first day of the week in user settings.