All year round, Sunday is public holiday in Greece. Shops, Supermarkets are closed, and public transportation bus and metro frequency is reduced. Exceptions apply to touristic attraction areas in Athens such as Monastiraki area, where touristic shops as those on Pandrosou Street are open during Sunday.
Regarding supermarkets, only the OK Anytime Supermarket chain is open on Sundays from 08:00 AM to 11:00 PM.
Public holidays in Greece - Wikipedia
Public Hollidays 2017