Once payment is made and Ad is created, your Ad is provisionally booked.
Your Ad would be released upon realization of your payment or receipt of your cheque number (if you've made your payment by a cheque).
As long as the calendar allows you to select a particular date, you can book your ad for release on that date. But it is suggested that you complete your booking and clear the payments at least 2-3 days prior to the release date.
ReleaseMyAd can create and broadcast your radio Ads if you book and pay 2 days in advance.