According to the FBI, more bank robberies happen on Fridays than any other day, and the most popular time for banks to be robbed is between 9 a.m. and 11 a.m.
Banks used to have more money on Fridays because, historically, it was payday. It’s probably still a perception, but it’s not necessarily true today.
As for 9 a.m. to 11 a.m., it’s the beginning of the banking day, and it’s perceived that there’s still a lot of money in the bank as opposed to the end of the day in terms of hard cash. Again, probably more a perception than a reality.