get weekday() {
    return this.isValid ? possiblyCachedWeekData(this).weekday : NaN;
  }
Get the day of the week.
1 is Monday and 7 is Sunday
@see https://en.wikipedia.org/wiki/ISO_week_date
@example DateTime.local(2014, 11, 31).weekday //=> 4
@type {number}