Most probably Samstagabend (der), depending on the context. Sonntag is Sunday.
If you want to say "I went out Saturday night" then you would say "ich bin am Samstagabend ausgegangen".
If you want to say "I could barely sleep Saturday night", then you would use Nacht (die) rather than Abend. "Ich habe waehrend Samtstagnacht kaum geschlafen."