Why not just send the invitation? You know the date, and presumably have the church/registry office/venue booked, otherwise you wouldn't be able to give that date for sure, so what other information do you need? Why pay for two lots of cards and two lots of postage? Complete waste of money. Especially in a recent case where someone I know sent out 'save the date' cards, then changed their minds about venue, couldn't get the new place on the old date so had to change the date anyway.