static getWeekendWeekdays({ locale = null, locObj = null } = {}) {
    // copy the array, because we cache it internally
    return (locObj || Locale.create(locale)).getWeekendDays().slice();
  }
Get the weekdays, which are considered the weekend according to the given locale
@param {Object} opts - options
@param {string} [opts.locale] - the locale code
@param {string} [opts.locObj=null] - an existing locale object to use
@returns {number[]} an array of weekdays, 1 for Monday through 7 for Sunday