Each year has 52 or 53 complete weeks. The year start or end is determined by applying the ISO Thursday rule to the week at the year border. If Thursday falls in the old year in the Gregorian calendar, the border week becomes part of the old year in the ISO calendar. If Thursday falls in the new year in the Gregorian calendar, the border week becomes part of the new year in the ISO calendar. For example, December 31, 2009, is a Thursday. Therefore, in the ISO calendar the week of Monday, December 28, 2009, through Sunday, January 3, 2010, is the last week in 2009. The first week of 2010 in the ISO calendar begins on Monday, January 4, 2010. In contrast, the Teradata calendar for 2010 begins on Friday, January 1.
2009 Dec last week ISO
2010 Jan first week ISO