Analti_Calte

The January effect has to be tradeable to disprove the weak form (and by extension the strong form, because the strong form by definition meets the criteria of weaker forms, plus additional criteria,  janakisri’s jargo-babble notwithstanding).  A pattern in prices does not disprove efficient markets if it is impossible to profit from it after transaction costs are considered. So it depends on whether the January effect is tradeable for a profit, which I think studies show it is.
Here’s an interesting data point. Virtually all of the growth in the S&P 500 over time comes from the overnight appreciation from close to open. If you could be in the market only during closing hours, you’d make a fortune with substantially less risk. However, you have to trade two times every day, buy at the close, sell at the open, and those transactions costs eat up all extra profits. Bummer, and that doesn’t provide enough evidence to challenge efficiency hypotheses, either.

anothercfainnyc

The answer is “it depends”! :-)