"Payable on" typically means that the payment is *received* on the day indicated. Thus, if support is payable on the first, the check should be in your ex-spouse's hands on the first of the month. It is not your issue or concern how long after that it takes for the bank to clear the check, unless you do something to delay clearing (insufficient funds; post-dated; put a hold or stop payment on the check; etc.); your obligation would be discharged by getting it to your ex-spouse on the first, which probably means mailing it at least three days prior. This doesn't mean your ex-spouse will take action if the check is a day or two late, just that you will technically be in violaton if that happens.