code: function calculateDatePos(startDayOfMonth, day) {
  	const start = startDayOfMonth - 1;
  	day = day - 1;
  	const weekDayIndex = (start + day) % WEEKDAYS;
  	const weekIndex = Math.ceil((start + day + 1) / WEEKDAYS) - 1;
  	return {
  		weekIndex,
  		weekDayIndex
  	};
  }
docs: Calcucalte given day's week and weekday index
  
  @param {number} startDayOfMonth
  @param {number} day