"Él camina los viernes."
Translation:He walks on Fridays.
The preposition is technically optional in English. But leaving it out is a much rarer form in spoken or written English. The fact that the Spanish leaves it out is irrelevant. You can't impose English structures, usages, and norms onto Spanish (or any other language).
It is just like that, except that in English we don't use the articles for the days (most of the time) and we also have different plural forms for the days. The construction is idiomatic. "Los viernes" includes the idea of "on" or another preposition that is appropriate from context.