Retail shops in Athens will operate in extended opening hours due to the Easter Holidays. The extended working hours start Thursday, March 29th 2018 and end on Holy Saturday, April 7th.
On Palm Sunday, April 1st, retail shops will be open.
The general plan is as follows:
On weekdays, Monday to Friday, from 09:00 to 21:00
On Saturday, March 31 from 09:00 to 18:00
On Palm Sunday, April 1, from 11:00 to 18:00.
On Good Friday from 13:00 to 19:00.
On Holy Saturday (April 7) from 09:00 to 15:00.
On Easter Sunday April 8th and the following Monday, April 9th shops are closed.
Check directly by supermarkets.
In Thessaloniki, shops will be open Monday to Thursday from 10:00 until 21:00; good Friday as in Athens, Holy Saturday until 16:00. Palm Sunday 11:00 to 18:00
I believe several regions follow different plans on holidays, so check directly.