Answered by Shaykh Faraz Rabbani
If a person performs ghusl the night preceding Friday (Thursday night) with the intention of making ghusl before Salatul-Jum’ah and then goes to sleep, would this count as having made ghusl before Salatul-Jum’ah or must it necessarily be made during the day on Friday?
Walakum assalam,
According to the relied upon position, it would not be sufficient to perform ghusl the night before Friday, because the sunna ghusl is for the prayer itself. As such, the complete reward is only attained if one goes to the Friday prayer in a state of purification that is due to that ghusl (and not a subsequent wudu). [Ibn Abidin, Radd al-Muhtar]
When it is not possible, due to constraints such as working on Friday, one may take the position that it is for the day. However, even according to this, one should make the ghusl after sunrise.
Wassalam,
Faraz Rabbani.