Why dec 31 2010 returns 1 as week of year?

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The definition of Week of Year is Locale dependent.

How it is defined in US is discused in the other posts. For example in Germany (DIN 1355-1 / ISO 8601): the first Week* of Year is the first week with 4 or more days in the new year.

*first day of week is Monday and last day of week is Sunday

And Java’s Calendar pays attention to the locale. For example:

public static void main(String[] args) throws ParseException {

    DateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
    Date lastDec2010 = sdf.parse("31/12/2010");

    Calendar calUs = Calendar.getInstance(Locale.US);       
    calUs.setTime(lastDec2010);

    Calendar calDe = Calendar.getInstance(Locale.GERMAN);       
    calDe.setTime(lastDec2010);

    System.out.println( "us: " + calUs.get( Calendar.WEEK_OF_YEAR ) ); 
    System.out.println( "de: " + calDe.get( Calendar.WEEK_OF_YEAR ) );
}

prints:

us: 1
de: 52

ADDED
For the US (and I can think of that it is the same for Mexico) the 1. Week of Year is the week where the 1. January belongs to. — So if 1. Januar is a Saturday, then the Friday before (31. Dec) belongs the same week, and in this case this day belongs to the 1. Week of Year 2011.

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