Week Number

The week number on this page follows ISO 8601, the international standard used across Europe, in much of the world’s business planning and in most software. Its weeks start on Monday, and week 1 is the week that contains the year’s first Thursday, which is the same as saying the week that contains 4 January.

That rule has two consequences people notice. A year has 52 weeks in most years and 53 in some, and the first few days of January can belong to the last week of the previous year, while the last few days of December can belong to week 1 of the next. The week table on this page shows the Monday and Sunday of every week, so the boundaries are never a guess.

Calendars set up the American way count differently: their weeks start on Sunday, and week 1 is whichever week contains 1 January, however few days of the new year it holds. In some years that gives the same numbers as ISO 8601 for everything except Sundays; in others, the US number is one higher for most of the year.

Neither is wrong, but a week number is only useful when both sides mean the same one. If a colleague, a supplier or a school quotes a week number, it is worth asking which rule they use, and checking which one your own calendar is set to.

The day of the year counts from 1 on 1 January to 365, or 366 in a leap year. The Julian date shown here is the form used on food packaging, in manufacturing and in military logistics: the last two digits of the year followed by the day of the year in three digits, so 23 September 2026, day 266, is written 26266.

Astronomers use a different Julian date, a running count of days from a starting point in 4713 BC. The page shows that one too, as the Julian Day Number; it is not the same number, so check which one a document means. The days left figure is how many days of the year remain after today, which is the number people usually want when they count toward the end of the year.