What is the last day I can pay my premium before the policy is canceled on my car? 
Technically this is set up by law in your state . It usually is a specified number of days after the normal expiration of the policy that would give you , and your lienholder notification of a cancellation . Many companies use a date prior to the expiration date as the date that the premium is due for this very reason . Usually the expiration date is close to scheduled expiration date . 