The odds for positive returns on the first trading day of a month

As I had mentioned previously the start of a new month is statistically the best day of the a month in terms of returns and today is no exception.  Currently as of mid-day most equity markets are up nearly 1%.  The Bespoke group specifically look at the opening trading day of may since 1992 and on average has returned 0.48% and has been positive 70% of the time.  Who doesn't like those odds?

What baffles me is that this is a somewhat known anomaly within the markets but yet it holds true month in and month out most of the time.

Data courtesy of Bespoke