Here is a list of national holidays in Greece, when almost everything (public services, shops, etc.) is closed.
Epiphany (6 January): Celebration of the baptism of Christ by St. John the Baptist, in the Jordan river.
Clean Monday (14 March): The celebration date changes every year. It is a day of fasting for Christians marking the beginning of Lent for the orthodox church as well as the ending of the Carnival season.
Feast of Saint George (23 April): Saint George is the patron saint of Arachova and is ordinarily celebrated on April 23. If the date occurs before Easter, the celebration of Saint George is postponed to Easter Monday.
Anniversary of the revolution of 1821 (25 March): Double celebration for Greeks, as the national revolution against the Turks in 1821, as well as the Annunciation of Virgin Mary, the announcement of the happy news by Archangel Gabriel to Mariam regarding the birth of Jesus Christ, are both celebrated on this day.
Good Friday (29 April): The celebration date changes every year. The day commemorating Jesus Christ's crucifixion.
Holy Saturday (30 April): The celebration date changes every year. The last day of Holy Week and of Lent. At midnight the Resurrection of Christ is celebrated.
Easter Sunday (1 May): The celebration date changes every year. The most important Christian celebration, celebrating the Resurrection of Christ.
International Workers day (1 May): Dedicated to the Chicago workers riot in May 1886 demanding an 8-hour work day and better working conditions.
Easter Monday (2 May): The celebration date changes every year. The second day of Easter.
Dormition of Virgin Mary (15 August): One of the most important orthodox celebrations, celebrating the passing and burial of Virgin Mary, with glorious ceremonies in many parts of Greece.
"No" Day (28 October): On this day the Greeks celebrate dictator Ioannis Metaxas's refusal, on 28th October 1940, regarding Italian claims, leading to Greece taking part in WWII.
Christmas (25 December): One of the most glorious celebration of Christianity, celebrating the birth of Jesus Christ.