checkForAllDayObjs(condition, filter) {
    return this.dayEls.every(d => {
      const dayObj = new DayObject(d);
      const dayNumber = dayObj.monthday;
      let shouldApply = true;
      if (filter !== undefined) {
        shouldApply = filter instanceof Array ? filter.includes(dayNumber) : filter(dayNumber);
      }
      // for instance, should be 'disabled' for the 15th and 20th day
      return !shouldApply || (condition(dayObj) && shouldApply);
    });
  }
@desc Applies condition to all days, or days in filter

@param {function} condition : condition that should apply for "filter" days
- Example: "(dayObj) => dayObj.selected"
@param {number[]|function} [filter] - month day numbers for which condition should apply.
- Example 1: "[15, 20]"
- Example 2: "(dayNumber) => dayNumber === 15" (1 based ,not zero based)