To me, your correction looks wrong. Here's why. 1. practice for when you get married one day. -> Correct. "when ..." makes a noun clause to serve as the object of "for". 2. practice for one day you get married. -> Wrong. "one day ..." is not a proper clause and can't come after "for". 3. practice for the day when you get married. -> Correct. "the day ..." forms a noun clause as in (1). It appears to me (1) sounds the best, but (3) is also correct and has a similar meaning.

"Practice for when you get married one day. " is a grammatical sentence - the other one is not grammatical, although there are various ways to fix it, such as "Practice for the day you get married"

I would use the first one, but the second one is correct also