The country marks the double holiday with two things: Parades and Bakaliaros (Cod fish).
What a beautiful day to celebrate a National Holiday! Sunny and not too warm! Greeks celebrate the 25th March a double holiday: a historical and a religious one. Greeks celebrate the War of Independence against the Ottoman Empire. The “Greek Revolution” was a successful war of independence waged by the Greek revolutionaries between 1821 and 1830.
Greek Orthodox Church celebrates the Annunciation by archangel Gabriel to the Virgin Mary that she should become the mother of Jesus Christ the Son of God. 25th March is exactly nine months before Christmas!
A custom across the country on this day is to eat crispy, fried Cod fish with garlic sauce (Bakaliaros skordalia)! This has to do with the Lent before Easter, where no animals or animal products should be eaten. However the Orthodox Church allowed an exception for the celebration of the Annunciation and that it the Cod fish!
source: Keep Talking Greece