In the Punch Card tab you can see a commit summary by day of the week and hour over the period starting from the first commit in this repository till now. For example, on the screenshot below the "Monday 12am" circle represents the total number of commits made on all the Mondays from March 22, 2006 till November 9, 2014 from 12 am to 1am.

Hovering your mouse over a circle will display the exact number of commits made during that hour on that day.