The same link, later doing a different job
This is the trick that makes one page serve two messages. Publish it now with the date and what little you know. When the venue, schedule and reply deadline are real, edit the same page and send the same link again. Nobody receives an outdated address, because the address was never frozen in a chat scroll.
The edit matters more than it sounds. Text-based save-the-dates get re-typed from scratch as the invitation, which is where dates get transposed. A page is updated once, and every tap afterwards, from either message, sees the current version.