"Ми будемо дивитися фільм у суботу."
Translation:We will watch a film on Saturday.
The sentence "We are watching a film on Saturday" is correct as well. The so called "futurate present progressive" (=Present Continuous) can be used to talk about á arrangement in the future that the speaker have already made (usually with somebody else, such as watching a film together next Saturday) It is explained very well at the Website below http://grammar.about.com/od/fh/g/Futurate-grammar.htm
Or more linguistically in the following paragraphes of the book Leech, Meaning of the English Verb §§98-101
These pages might be accessible over google books: https://books.google.at/books?id=ryasAgAAQBAJ&pg=PA69&lpg=PA69&dq=futurate+present+progressive&source=bl&ots=R2Q37k7EAI&sig=3yAfvLTLNNqcbORKPlmgirlnHgk&hl=de&sa=X&ved=0CDIQ6AEwAmoVChMI78-v6LLxxgIVSFcsCh3AbwDe#v=onepage&q=futurate%20present%20progressive&f=false