Here is a list of the national holidays in Greece, when almost everything is closed.
New Year’s Day (January, 1st)
Epiphany (January, 6th)
Ash Monday (February/March)
War of Independence National Holiday (March, 25th)
Holy Friday (March/April)
Holy Saturday (March/April)
Easter Monday (March/April)
Labour Day (May, 1st)
Monday of the Holy Spirit (7 weeks after Easter) *
Dormition of the Virgin (August, 15th) *
Ohi (No) Anniversary (October, 28th)
Polytechnic School Uprising (November, 17th) *
Christmas (December, 25th)
Synaxis of the Virgin Mary (December, 26th)
* During Monday of the Holy Spirit, Dormition of the Virgin Mary and Polytechnic School Uprising holidays most shops remain open.