Is it against the law to repo someones car if there barely a day late?

This might help you to visualize this better.

Let's say the car payment was due on the 10th of the month. At midnight, you still have not made that payment, and at 12:01 am on the 11th, the repossession agent hooks up to your car in your driveway and drives away. No, it is not against the law, PROVIDED:

1. The lender has contracted the agent to recover the vehicle.
2. The lender has provided a legal order of repossession.
3. And, the repossession agent has followed the repossession and collection laws of the US and the state.

It is not likely that this would occur though. The logistics of the situation take time. This is part of the reason most lenders have a five to ten day grace period.

The more likely scenario is that the payment was due on the 10th, was not paid, the five day grace has come, and the payment has not been made. During the five days since the 10th, the lender has contacted the repossession agency, has sent them an order for repossession (electronically), the account has been entered into the repossession agencies system, the account has been assigned to an agent, and at 12:01 am on the 16th, the repossession agent secured the car in your driveway and affected recovery.

Your car being repossessed is not the fault of the lender or the repossession agency in most cases. In the majority of cases it is the fault of the borrower, either from personal failure or unforeseen circumstance.