you will have to start it all again she should at least have it done a couple of days before she was done last year i know that because my friend missed his by 2 days and she had to start a new course!
are you talking about flu, if so thats got to be done yearly without fail otherwise you have to start the course again,,, if however its tet then is diff i was told by a vet can be done once every 2 years or once a year, have a chat to your vet!!
If it's just tetanus, then you will still be covered if you go over the booster date, which is every two years after the initial course. If it is flu & tet, then for the certificate to be "show legal", you will have to start the whole course again.
The time when it will be checked for every date would be at Riding Clubs Area Qualfiers and Finals. Every single horse is checked. Some POny Clubs check for camp too.
Local shows and even County Shows do not often enforce the rules,