Every week yr is in sync with a WEEK_OF_YEAR cycle. All weeks between the first and final weeks (inclusive) maintain the same week yr cost. Therefore, the first and final days of per week yr can maintain various calendar yr values.
For instance, January 1, 1998 is a Thursday. If getFirstDayOfWeek() is MONDAY and getMinimalDaysInFirstWeek() is 4 (ISO 8601 fashioned appropriate environment), then week 1 of 1998 begins on December 29, 1997, and ends on January 4, 1998. The week yr is 1998 for the final three days of calendar yr 1997. If, on the opposite hand, getFirstDayOfWeek() is SUNDAY, then week 1 of 1998 begins on January 4, 1998, and ends on January 10, 1998; the first three days of 1998 then are portion of week 53 of 1997 and their week yr is 1997.