Congratulations! (It certainly is a relief, isn't it?!) :)
If you're talking about the letter from the Home Office, don't worry too much about that date; I didn't even get my letter till a day or two before the ceremony (which had been booked several weeks before!). Each registry office does things its own way, anyway -- and many of them will send you their own invitation.
What happens is this: the Home Office transmits your information by email to your local registrar, and also dispatches the hard-copy and, I think, their own invitation letter on the same day. Your local registrar will not be able to book you in for a ceremony at least till they receive the email notification; but by the time you get a Home Office invitation letter, the registrar should have your details in one form or another.
The important thing isn't the booking-date, though; it's this: you have to attend a ceremony and be sworn in within **90 days** of the date of your approval. As long as you do that, you're o.k.
Just ring your council tomorrow and don't worry. If it's really looking like a problem by the end of the week, ring the Home Office and make sure they help sort it out.
Good luck.