You will be charged weekly, starting with the day your reservation begins. So if you start your reservation on a Tuesday, you will be charged every week on Tuesday. You are also charged for the day you return the car.
If you decide to renew your reservation, or change cars, the day you renew or change will become your new payment day (so if you renew on a Thursday, your new payment day will be Thursdays).
Watch this video to learn more about payments and invoices.