In the days following, the word of Metaxas’ denial had spread around the capital and the Greek population took to the streets shouting “Oxi!”. His decision made on 28th October 1940, is commemorated each year as a day that represents bravery, solidarity and heroism for millions of Greeks all around the world.” (derived from this source)
It is usually celebrated with processions of students–and the top student of each school is the flag bearer at the head of the procession, as well as military processions in Athens and Thessaloniki. It is a National Holiday, and all schools, state employees and most businesses are closed–schools usually have the eve off as well to prepare for celebrations.