Maybe there was an announcement that I missed or maybe GOG is late in posting it, but I was looking forward to the Weekend Promo. Was it cancelled because of the Battle Of The Games Summer Promo and, if so, does that mean there won't be one next weekend as well?

Very likely. I suspect instead of one set of games discounted at 50% or so over the whole weekend, we'll get two games discounted at 40% and 60% for each day, including Friday, Saturday, Sunday and Monday. In the end that's 8 games discounted at 40-60%, not much unlike a weekend promo, with the difference that it changes every day.

From twitter

GOGcom ‏@GOGcom

No Weekend Promo today; wait 2 hours for the next round of Battle Of The Games #BOTG http://j.mp/BOTG_GOG