Most of the evidence for these effects is anecdotal, although there is a slight statistical case to be made for some of them, which is more than enough to encourage some investors to place their faith in them.
Proponents of the October effect, one of the most popular theories, argue that October is when some of the greatest crashes in stock market history, including 1929's Black Tuesday and Thursday and the 1987 stock market crash, occurred. While statistical evidence doesn't support the phenomenon that stocks trade lower in October, the psychological expectations of the October effect still exist.
Investment dictionary. Academic. 2012.