interesting investing problem

Okay, I'm setting up my Vanguard account to do weekly investments in the market.

They ask me what day of the week I'd like my investments done on.

That's an interesting question. I want to buy on whatever day the market is most likely to be down, because buying low is better. Would people sell more often on Mondays? Maybe Fridays?

There's got to be a way to check this. Get statistics on market performance and evaluate what day is statistically the worst for the market, and buy on that day. Has anyone ever seen data on this already collected, or heard a report on the news or anything?