An important source of the month-to-month variation in many monthly economic time series is trading-day variation. In activities such as production, sales, and shipments in domestic and foreign trade, the monthly rate of activity is related to the number of working or trading days in the month. A familiar example is retail sales, where more sales are made on Fridays and Saturdays than on other days and higher sales are made in months containing five Fridays and/or Saturdays than those with four.