What kind of summer sale is this without Stardew valley

There is a possibility that the publisher had previously agreed to discount the game during an as of yet unknown upcoming period. I have noticed that some of the best prices appear during miniature promotional events (prior to, or following major sales).

Based on pricing history provided by GOG Database, it would seem that there is a decent chance that Stardew Valley will be placed 'on sale' in the not-too-distant future. However, this is purely speculative.

Depends on the publisher not GOG. Usually it is on sale every few months. Put it in your wishlist and keep checking every day (not week because sometimes sales end in less than 7 days)

You can see Stardew Valley latest - quite irregular - sales on GOGDB (a third-party website):
https://www.gogdb.org/product/1453375253

It is on sale now if you are still interested :)

(Sale ends July 10)

Right on!