Usually make ghusl the night before, because the Arabic day starts at sunset, and you want to pray special duas after Maghrib and Isha prayers, before you go to sleep that night. When you wake up, on Arbaeen Day, make ghusl again if you need to before Fajr prayers. If you want to make a special ghusl, you need to do it before you pray Zuhr and Asr prayers on Arbaeen Day, because Imam Hussein AS was martyred afternoon on Arbaeen Day.