It is well known that the Greeks are excellent merchants. Many neighbor countries are coming to Greece because of shopping. For most of tourists, one vacation can not be finished without at least one day of shopping, especially if it is the period of sales. Many tourists are planning their vacation according to the dates of sales.
The minimum discount is 30%, but some things are even 70 to 80% discounted.

In general, the periods of sales are, more and less, the same every year and there are four in total, two summer and two winter. During those periods the shops are usually open even on Sunday.

Winter discounts in shops in Greece always start on the second Monday in January. The difference from year to year can be a few days.
The discount always lasts until the end of February, which means February 28 or 29.
Also, there is a shorter discount (ten-day discount) that is always in the first ten days of November, from November 1 to 10.
The 2024 Winter Sales start from Tuesday 2nd January 2024 and run until and including Saturday 27th January 2024.
Summer discount periods are in May, July and August. The first period is always in the first ten days of May, and the second begins on the second Monday of July and lasts until the end of August. 
You can remember this as a general rule that is always followed.
 The 2024 Summer Sales will run from Monday 24th June to Saturday 20th July 2024 inclusive.

On the very beginning of the sales there is the largest selection of clothes and footwear, while at the end is the smallest, but often the unsold clothes and footwear are additionally discounted in order to be sold. In that period usually only large numbers of clothes and footwear are left.

Also, it happens that some shops low the prices few days before the official beginning of sales. It can be followed on websites of some shops, especially if you already now what you want to buy.

In Greece a large selection of clothes and footwear with reasonable prices can be found even when there are no sales.

In the periods of sales, also the household equipment is discounted, not only clothes and footwear.

The working hours of shops in Greece you can check here.