Start of week for locale using Joda-Time

Joda-Time uses the ISO standard Monday to Sunday week.

It does not have the ability to obtain the first day of week, nor to return the day of week index based on any day other than the standard Monday. Finally, weeks are always calculated wrt ISO rules.

Seems like you're out of luck, it looks like all of the provided Chronologies inherit the implementation from baseChronology, which supports only ISO definitions, i.e. Monday=1 ... Sunday=7.

You would have to define your own LocaleChronology, possibly modeled on StrictChronology or LenientChronology, add a factory method:

public static LocaleChronology getInstance(Chronology base, Locale locale)

and override the implementation of

public final DateTimeField dayOfWeek()

with a re-implementation of java.util.Calendar.setWeekCountData(Locale desiredLocale) which relies on sun.util.resources.LocaleData..getCalendarData(desiredLocale).

There's no reason you can't make use of the JDK at least to find the "customary start of the week" for the given Locale. The only tricky part is translating constants for weekdays, where both are 1 through 7, but java.util.Calendar is shifted by one, with Calendar.MONDAY = 2 vs. DateTimeConstants.MONDAY = 1.

Anyway, use this function:

/**
 * Gets the first day of the week, in the default locale.
 *
 * @return a value in the range of {@link DateTimeConstants#MONDAY} to
 *         {@link DateTimeConstants#SUNDAY}.
 */
private static final int getFirstDayOfWeek() {
  return ((Calendar.getInstance().getFirstDayOfWeek() + 5) % 7) + 1;
}

Add a Locale to Calendar.getInstance() to get a result for some Locale other than the default.

Here is how one might work around Joda time to get the U.S. first day of the week:

DateTime getFirstDayOfWeek(DateTime other) {
  if(other.dayOfWeek.get == 7)
    return other;
  else
    return other.minusWeeks(1).withDayOfWeek(7);
}

Or in Scala

def getFirstDayOfWeek(other: DateTime) = other.dayOfWeek.get match {
    case 7 => other
    case _ => other.minusWeeks(1).withDayOfWeek(7)
}