Public Holidays

New Year's Day: January 1st
Epiphany: January 6th. The seas around Piraeus are blessed. A cross is thrown into the sea and young men dive to retrieve it.
Ash Monday: 41 days before Easter. Lent begins. On Ash Monday, Greeks fly kites, eat Lenten food and celebrate the koulouma.
Independence Day and the Feast of the Annunciation: March 25th , Military parade.
Easter: Good Friday-Easter Monday. Vesper evening is on Good Friday and every church decorates an Epitaphios (bier). The Epitaphios processions, followed by people holding lit candles and chanting hymns, fill the streets of every city, town and village in the country.
Anastasi (Resurrection): celebrated with fire-works and lit candles at midnight, on the eve of Easter Day.
Pascha (Easter): on that Day Greeks traditionally eat lamb, spit-roasted on charcoal. The festivities include singing and dancing through the day.
Labour Day: May 1st . Many Flower Festivals take place all over Greece.
Whit Monday: 50 days after Easter.
Dormition (Assumption) of The Virgin: August 15th
September 27: My birthday. It is not a public holiday but since this is my website I guess I can include it.
October 28th: rejection of the Italian ultimatum in 1940. Military parade.
November 17th: Commemorates the squashing of the student rebelion at the Politechnic University in 1973 by the Greek dictatorship with a march and stoning of the US embassy and battles with riot police. More of a 'people's holiday' than a public holiday but I have to mention it since it can affect some travel plans.
Christmas: December 25th -26th.

See Also Holidays in Greece which has more holidays and more detailed info