I only have a few spots left for Christmas and I typically book out 4-6 weeks.
The exception is a newborn. A newborn is only a newborn for a very short time, a few weeks at best.
I can always fit in a newborn. They are like Jello - there is always room for Jello.
If you call me when you are in labour or shortly after - I promise you I will fit you in with 10 days. This does not apply after your baby is three weeks old.
Hint: A session makes a really really good baby shower present. ;)