ciemnogrodzianin: In case of Insomnia - you're right. I thought there wasn't this kind of sale last year, but it's not true.
Here is a list of sales with dates. It seems that usually we can expect Insomnia in March.
adaliabooks: Yeah, it's no guarantee they will have another but as Insomnia is a big deal in the Gog community I can't see them dropping it completely (although I hope if they do have one it's a little more interesting than the previous two have been, the last one was particularly poor)
Double Insomnia (March '15) was a great sale, I thought, though it was the first such sale since I signed up, so I had not much frame of reference. :) But I'm pretty sure it had plenty of bigger titles for north of 70% off (I picked up
Terraria for a rare 80% off, for example, and
Banished for a then-high discount of 70%), and it was the last Insomnia to do the "click 'Buy now' and get a chance to get the game for free" lottery (which netted me 3 games!). Also, the fact that it featured two games in the flash discount section -- one "older", one "newer", basically -- made it a little more likely that one would happen to find something that interested them (though there were always the lower "normal sale" discounts to fall back on if the featured deals of the moment didn't trip one's trigger; I don't recall if last year's Insomnia even had the other, not-currently-featured stuff discounted at all).
Yeah, the Sleepy Sheep Insomnia Sale (March '16) was pretty dull, and had some bizarre artificial restrictions: no direct way to go to a featured game's store page, only to "buy now"; some weird restriction regarding store credit (either you were forced to use it first, or you couldn't use it at all -- didn't affect me, so I can't remember which). The mostly unremarkable flash discounts you mentioned in your other post were definitely a thing in this sale, coupled with even smaller quantities of most games than in previous Insomnia sales...though that at least kept less-desirable games from forming an hours-long logjam like in those prior Insomnias.