Yes, we don't typically say it in the present if we are speaking about something in the future, unless like you said, you used the plural like your example above. It makes more sense in English to say I WON'T be here on Saturday. I know that in Hungarian the literal translation of that would use leszek, but that would be if you were translating from English to Hungarian. But if you take that Hungarian sentence, "Szombaton nem vagyok itt.", the best translation into English would be I won't be here on Saturday.