Start and end dates for the Steam Halloween, Black Friday and Winter sales have been leaked on Reddit, and we've independently corroborated them as correct.

The remaining Steam sales in 2017 are:

Steam Halloween Sale: Thursday 26th October to Wednesday 1st November

Steam Black Friday Sale: Wednesday 22nd November to Tuesday 28th

Steam Winter Sale: Thursday 21st December to Thursday 4th January 2018

Remember, Steam no longer does flash sales so discounts last the duration. Also remember there will be rival sales at places like GOG during similar periods. Spend wisely.