Often people will say that the first of the month tends to be positive. By the above analysis (SPY from 07/01/2000 to 06/13/2014), that has--on average--been the case. You could also say go long for the 9th trading day of the month and short for the 14th. And if there are 23 trading days in the month, go long the close of the day before--well, if it happens to be in August.
Good luck and good trading. Please do not use the above for trading. Prepare your own analyses (the above could be flawed) and do in-depth investigations. I am not giving financial advice. I'm merely playing with numbers.