Seizures in dogs can have a variety of causes. The most common cause is epilepsy, as it is in humans, but there are several medical conditions which can cause seizures, like diabetes, and also, ingestion of a toxic substance can cause seizures, as well.
The best thing for your to do is have your dog evaluated by the vet, have some lab tests done, and he'll be diagnosed, then treated, Some seizures are 'idiopathic', meaning they have no known cause, but they can still be treated with an anti-convulsant medication.
If your regular vet is closed now, please bring him to an ER vet center to be evaluated and treated. If you have no ER vet center in your area, look in your local Yellow Pages under 'Veterinarians' and find listings including 'Emergency Hours' and/or 'Emergency Phone Numbers'; start calling vets in your area until you find one who will come into the office for you or who will make a housecall.
For the moment, monitor him closely, as you have been, and don't let him jump up on anything, as he may fall if he convulses again. Keep him safe on the floor, and try your best to get him seen.
I hope all will be well!