Is Steam sale date confirmed, then?
Neither Steam nor GOG sales' dates have been ever confirmed. It's always business secret.
About the predictions:
- in case of Steam it is always known before, probably they have discipline issues with hundreds or thousands of publishers, some of them cannot even make games ;)
- in case of GOG it's only a guess and we can never be sure, but recently they tend to start their sale before Steam one, which is quite reasonable I think if you're competing for the same wallets.