"Goodbye, until tomorrow!"
Translation:Do widzenia, do jutra!
In this phrase, the word "tomorrow" is treated as a noun (just like in "until Friday") and undergoes declension - it takes Genitive because of the preposition "do".
Firstly, it's "cześć", secondly it would be too informal for "goodbye", thirdly we don't even have "goodbye" here but something more specific: "until tomorrow".
Sometimes, the English language really isn't the best instrument for learning Polish. In other languages such as German (Auf Wiedersehen) and French (Au revoir) there are expression similar to Polish 'Do widzenia', unlike English where 'See you' is for the literal sense but rather informal and 'Goodbye' for the formal, but not with the literal meaning of seeing the other person(s) some time again..