Learn French: Samedi, on va au cinéma. - Saturday, we are going to the movie theater.

If you just say the day of the week like samedi, it means this coming Saturday. Since this lesson is about habits, remember that adding le (like le samedi) changes the meaning to every Saturday. Because they are just going to the movies this one time, we leave out the le.

The translation says "we", but doesn't on usually mean "one"?

Yes, on traditionally means "one", but in casual spoken French, it is almost always used to mean "we". Just remember that even when it means "we", it still uses the same verb form as il and elle (he and she). That is why we say on va instead of using the nous form.

Why is it au cinéma instead of à le cinéma?

In French, the preposition à (to/at) and the masculine word le (the) can never stand next to each other. They always squish together to form the word au. Since cinéma is masculine, "to the movie theater" becomes au cinéma.

Why isn't there a word for "are" in the French sentence, like "we are going"?