Learn Russian: В выходные не надо напрягаться. - On the weekend there is no need to strain oneself.

This ties into the main rule of negative obligations! After не надо (there is no need to / one shouldn't), we almost always use the imperfective infinitive. Using the imperfective here expresses that the action of stressing out is generally unnecessary and shouldn't happen at all.

What is the literal meaning of напрягаться, and why does it end in -ся?

The base verb напрягать means "to strain" or "to make tense". Adding the reflexive ending -ся makes the action affect the subject, so напрягаться literally means "to strain oneself" or "to make oneself tense". In everyday conversation, it is a very common way to say "to stress out" or "to push oneself too hard".

Why is выходные in the plural here?

In Russian, the word for "weekend" (выходные) is essentially an adjective meaning "days off" that functions as a noun. Because a weekend usually consists of two days, it is almost always used in the plural.

What case is В выходные and why?

It is in the accusative case. When we want to say "on" a specific day of the week or weekend, we use the preposition в plus the accusative case. Since выходные is an inanimate plural noun, its accusative form looks exactly the same as its nominative dictionary form.