It's more or less up to you. I am from the former "East" and we used Sonnabend. I think it strongly depends on the area you live in. In southern Germany they use "Samstag". We live in Berlin and my daughter is always correcting me when I say Sonnabend, telling me it should be Samstag. Maybe there is someone from South/West-Germany who can tell if it's true, that the Samstag is more a West-German thing...

Both are correct, "Samstag" is more common.