Smallpox was an ancient evil that killed 500 million human beings. We destroyed it. It is one of if not the most impressive and impactful thing our species has ever done and deserves to be recognized as an international holiday of the highest order.
The World Health Organization declared Smallpox officially eradicated on December 9, 1979. Five months later, the World Health Agency formally declared Smallpox eradication on May 8, 1980. There wasn't a procedure in place for deciding when to officially recognize such a momentous event, as nothing like it had ever happened before or since. Feel free to celebrate as much as you want on either day, or any other day for that matter. Personally, although I used to favor December 9th, I've come to think of May 8th as the main Smallpox Eradication Day. December has enough going on and May needs more holidays.