Average amount of cars/day if fine=YES

Sorry guys, i'm new here: I want to see the average amount of cars that get a fine per day in my dataframe. So i want to make an average of the cars that get "Fine=Yes" per day. How can i do this?

df Car Fine Date 1 Yes 10/10/2017 2 Yes 10/10/2017 3 Yes 10/10/2017 4 No 10/10/2017 5 No 11/10/2017 6 Yes 7 No 8 Yes 9 No

Thanks