[Question] What is the best day to celebrate Smallpox Eradication Day?

Several of my friends posted on Facebook/​threw parties this weekend for a date of May 8. However, searching on Facebook, I find that I and several other friends have been posting celebratory messages on Facebook on December 9 in previous years.

This inconsistency is probably because there are two obvious candidate dates for eradication:

The global eradication of smallpox was certified, based on intense verification activities, by a commission of eminent scientists on 9 December 1979 and subsequently endorsed by the World Health Assembly on 8 May 1980.

500 Million, But Not a Single One More”, which is widely shared and has been adapted at least once for reading at a large Secular Solstice, uses the December 9 date.

I think creating more consensus on which date is the “official” one for celebration purposes would be a small improvement (at the very least: I would stop confusing my relatives by year-to-year inconsistency on when I message them a little commemoration). So, which is it?