"Milyen nap van ma?" means "Which day of the week is it today?", it's a fixed expression.
Actually you're right, the sentence literally means "What is the day like today?". But nobody would answer: "It's a beautiful, sunny day.", everybody would say "Wednesday". You should change the sentence depending on the context, so that it's not the fixed expression any more. For example: "Milyen ez a mai nap?".
"Which day is it today?" would be literally "Melyik nap van ma?" but if someone would ask me this, I would have no idea what he wants to know.