Your first week’s subscription is charged before you pick up the car, as soon as you suscribe and covers your first week of driving. Subsequent subscription payments will be charged 1 day before your weekly subscription renews.
You make and pay for your subscription on Monday 19th March, but choose for your subscription to start on Wednesday 21st March. (This is the day that you would pick up the car). The payment you made on Monday covers you for your first week Wednesday 21st March - Wednesday 28th March.
So, your subscription weekly cycle is from Wednesday-Wednesday. The payment for next week (w/c Wednesday 28th March), will come out 24 hours before your subscription renewal date, on Tuesday 27th March.