Using daily returns from the NASDAQ Composite and TSE 300 Composite indices from 1984 to 2003, we specify a method that corrects the chaotic forecasting of financial time series taking into account the day-of-the-week, the turn-of-the-month and the holiday effects. When calendar effects are present in the series, the forecasting ability of the model leads to profitable opportunities compared to a buy-and-hold strategy.