Delixe: Croteam were probably hinting that all Serious Sam games will be 50% but there will be a daily and that will probably drop to your 75%.
Pheace: Yeah that's what I meant to say in the post above (but in a really confusing way xD)
That combined with the Midweek deal this week abruptly ending when the Weekend deal normally starts, rather than overlapping for ~6 hours more seems to be a good indication of the Summer Sale hopefully starting Thursday.
Basically threw off other retailers probably (since these things tend to be planned far in advance so they might have planned around it somewhat), and the trick with giving out TF2 for free when the last Summer Sale started and then throwing a Summer Sale at us a week later would be pretty brilliant (on their part)
Basically they are trying to make people not buy anything during others' sale, waiting for steam sale. That's not just brilliant, it's basically ruthless and very jackassery. They don't just want to sell much, they want others to sell nothing.
Of course this has no proof, so it's just a stupid theory on my part. :p