Will you go to a sauna tomorrow should be accepted.
"Are you going to the sauna tomorrow?" = should be accepted. It's more natural way asking someone about it.
Heh heh. In Canadian-Ukrainian (or at least how I learned it), лазничка is the word for bathroom.
There is a resort in Czechia called Marianské Lazně, close to the border with Germany. Different country, but similar Slavic language!
Ти підеш до лазні завтра?