You are not allowed to post-date a check, or to date it for a date in the future (even if it's only a few days in the future), unless the other side specifically agrees to it. Otherwise, they may deposit a check when they receive it. It is your responsibility to manage your budget: you have to make sure that you have sufficient money to cover the payment when the finance company receives it, while mailing it with sufficient time to make sure they get it on time.