on Monday, 29 December 2014 and ended on Sunday, 4 January 2015, because 1 January 2015 fell on Thursday. Week 1 of 2021 (2021W01 in ISO notation) started on Monday, 4 January 2021 and ended on Sunday, 10 January 2021, because 1 January 2021 fell on Friday. Week 52 and 53 It is also possible to determine if the last week of the previous year was Week 52 or Week 53 as follows: If January 1 falls on a Friday, then it is part of Week 53 of the previous year (W53-5). If January 1 falls on a Saturday, then it is part of Week 53 of the previous year if that is a leap year (W53-6), and part of Week 52 otherwise (W52-6), i.e. if the previous year is a common year. If January 1 falls on a Sunday, then it is part of Week 52 of the previous year (W52-7). Schematic representation of ISO week date Notes 1. Numbers and letters in parentheses, ( ), apply to March − December in leap years. 2. Underlined numbers and letters belong to previous year or next year. 3. First date of the first week in the year. 4. First date of the last week in the year. Other week numbering systems In some countries, though, the numbering system is different from the ISO standard. At least six numberings are in use: Because the week starts on either Saturday, Sunday, or Monday in all these systems, the days in a workweek, Monday through Friday, will always have the same week number within a calendar week system. Quite often, these systems will agree on the week number for each day in a workweek: In years where 1 January is a Monday, Tuesday, Wednesday, or Thursday, all of the above week numbering systems will agree. In years where 1 January is a Friday, ISO-8601 will be different, but the rest will agree. In years where 1 January is a Saturday, ISO-8601 and the Middle Eastern system will agree, being different from Western Traditional and the Broadcast Calendar which will agree. In years where 1 January is a Sunday, the Broadcast Calendar will be different, but the rest will agree. Note that this agreement occurs only for the week number of each day in a work week, not for the day number within the week, nor the week number of the weekends. The epi