"We will go on Monday."
Translation:हम सोमवार को जायेंगे।
It's just something you pick up on with familiarity. Because आना and जाना are intransitive verbs which cannot take a direct object, one would think you would always need the को. But it is a quirk of the language that when the noun is कल, परसों, सुबह etc or locations like घर, city names etc we skip the को but act like it is still there (using the oblique case for the noun).