Until a sale is announced no one knows when it will be. All official sales (so Steam and Paradox Plaza) get announced on the official social media channels, so you can follow those to make sure you won't miss any.
https://www.facebook.com/CitiesSkylines
https://www.twitter.com/CitiesSkylines
https://www.facebook.com/CitiesSkylines
https://www.twitter.com/CitiesSkylines