There are no laws or regulations that address when a federal credit union may repossess your vehicle. Technically, if your payment is one day late, you have defaulted on your loan.
Each credit union may set its own policy on the conditions of default that will trigger repossession. These conditions of default are included in the loan agreement, which is the contract governing your loan. When you accepted the loan, you accepted these conditions. You should contact the credit union if you have questions about the terms of your contract.