According to the Dev diary of the CEO of Artery Studios (Secret of the Magic Crystals) here are the possible dates of the next two major Steam sales.
Autumn (Nov 21 – 27)
Winter (Dec 20 – Jan 4)
In addition, according to CDR there’s possibly a THQ Humble Bundle in the works. CDR is a site that tracks updates to the Steam database. I believe that’s how someone figured out what the Summer Indie bundles were going to be.