Dec, Mar, June, Sept are good months to buy. Jan, April, July, Oct are good months to sell.
Feb, May, August, November, are good months to stay out. Rationale is quarterly earnings are released in these months. Sell on good news often happen, market is choppy due to influx of good and bad earnings reports.
Dec, Mar, June, Sept, after the down cycle of reporting season, is good time to enter at low. Which coincides with end of month reporting by funds. Window dressing at play. Jan, April, July, Oct, after prices rally for a while and reach upper range, its good time to take profits.