Usually this occurs at the end of the sale.

There will probably be more cool sales by now and next summer.

Nah, there have been mid-of-the-sale days from previous sales (summer and winter) in response to Steam starting their own summer/winter sale.

WIth the normal summer and winter promo and some others, GOG usually has 2 "all-in" days throughout the promotion where all bundles etc. from the entire promotion time period show up all at once for 24 hours. They usually do this half way through the promotion (as they did a day or two ago) and then again on the very last day of the promo as an encore "last chance" type sale. I like this format and it seems to be well liked by the majority of people too so they'll probably stick to the format I imagine. We can never know for sure until they do stuff but there's enough history that it's a reasonable assumption to make anyway.