Valve never announces Steam sale dates up front, but Steam sales return every year in a relatively stable frequency. Based on data from the last few years, the upcoming sale dates are predicted. These predictions are a combination of the following factors: Dates and months in which a sales started.
Steam Winter Sale 2018 dates and times. Earlier this year, Kotaku reported that a developer had shared the dates for its Winter and Autumn sales. This report was correct with regards to the dates of the Autumn sale, so with luck they’re accurate on the Winter sale too.