Do I pay my debt payment at the beginning or ending of the due date?
On the 8th day (or whichever day your next payment is due), you will have the entire day to use the time to do whatever you need to in order to make the payment. At the end of the day is when Tear will ask you to pay up, and you'll either successfully make the payment and continue through the game, or end up living in a cardboard box.
I'm not joking. Most guilt-inducing Game Over screen ever.