day method
Return a day of the week as a DateTime object based on the provided day index. Day 0 is Monday.
Implementation
DateTime day(int day) {
// According to ISO the Jan 4th must be in week 1
final addDays =
(weekNumber - 1) * 7 + (-DateTime(year, 1, 4).weekday + day) + 1;
return DateTime(year, 1, 4).addDays(addDays);
}