I have a litter of Siberian kittens, and I'm thinking of taking them to a cat show in a couple months, but they'll be at the exact age when they are supposed to get vaccinated at the time. Polyvalent vaccine should be fine -- if I vaccinate them at 9 weeks and re-vaccinate 3 weeks later, they'll have exactly 3 weeks till the show. But then there's the rabies vaccine. All of the kinds I can find locally state that it should be administered to at least 3 months old kittens. If I vaccinate them 3 weeks before the show, they'll be 12 weeks old, which is about 10 days younger than 3 months. Are those 10 days that important? What will happen if I vaccinate the kittens before they're ready? And is there a way to determine if they're ready to be vaccinated other than their age?
Blackwell's 5 minute Veterinary consult lists the starting age of vaccination for rabies at 12 weeks. Purevax Feline Rabies vaccination is listed as suitable for felines 8 weeks or older.
Long and the short of it, you should be good to vaccinate. As a rule of thumb, I generally find that "months" when used to describe things like medical dosages to be very imprecise, weeks or days is always better. When I see "months" used then I'm almost always sure it's in 4 week increments, but always good to confirm.