Do you know when is it “week01” of 2010?

We run our project and release calendars based on week numbers. Taking into account that we run as an agile organisation with 2-weekly cycles, it helps to remember that things normally happy every odd or even week.

I was trying to work out the PDK release schedule for the next few months when I stumble at the oddity that are 2009 and 2010.  My problem was that 2009 has 53 weeks! and clearly 2010 starts with week 01. This creates the unusual situation of having 2 odd weeks in a row…

Ok, I can live with this… I said to myself 🙂 but then I relaised that my calendar was telling me that 1st Jan 2010 was part of week53 2009… that makes no sense, right?

Don’t worry help is at hand, our friends from ISO have develop a calendar standard that this with this situation –ISO8601. So it turns out that according to wikipedia:

There are mutually equivalent descriptions of week 01:

  • the week with the year’s first Thursday in it (the formal ISO definition),
  • the week with 4 January in it,
  • the first week with the majority (four or more) of its days in the starting year, and
  • the week starting with the Monday in the period 29 December – 4 January.

If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, Saturday or Sunday, it is in week 52 or 53 of the previous year (there is no week 00). 28 December is always in the last week of its year.

What a shocker! now that I know, I can sleep better at night …

2010 calendar from

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