Quote from gmst:
which instruments did you test ?
Btw, gap stats, especially when overlapping week of day effect will have very small sample size, especially if you did it only for one instrument. Its very easy to see pattern in randomness. Be very very careful.
I am saying above based on my below experience.
I did a comprehensive study on daily returns on equity indices (SP) for last 20 yrs, and found no evidence of so called Monday effect. I concluded that weekday anamoly doesn't exist. Interestingly, my first impression after crunching the numbers was that weekday anamoly does exist, but on much closer looking into data, I changed my conclusion.