Should the Friday (Ghusl) it should be on Fajr prayer or on Zuhr prayer?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The Ghusl for the Jumu'ah prayer is desirable and not mandatory, according to the opinion of the majority of the scholars including the four eminent Imams.

The time for performing the Jumu'ah Ghusl extends from the Fajr prayer time until the Jumu'ah prayer time. But it is more desirable to perform such Ghusl just before going to the Jumu'ah prayer.

Allah knows best.