Week numbers are useful for scheduling and planning. ISO 8601 defines a week as beginning on a Monday (Day 1) and ending on a Sunday (Day 7). A year can have 52 or 53 weeks. Week 1 of the year is defined as the week containing the 4th January or equivalent definitions such as the week containing the first Thursday of the year. The complication is that the 1st/2nd/3rd January can fall in Week 52 (or 53) of the prior year and the 29th/30th/31st December can be in Week 1 of the following year.