In أَمَّا يَوْمَ الْجُمُعَةِ فَزَيْدٌ مُنْطَلِقٌ, the fronted phrase yawma l-jumuʿati is a temporal adverbial, not a nominative subject.
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The Following Noun Is Not Always Mubtadaʾ
Conditional Particles: In, Law and Ammā
In أَمَّا يَوْمَ الْجُمُعَةِ فَزَيْدٌ مُنْطَلِقٌ, the fronted phrase yawma l-jumuʿati is a temporal adverbial, not a nominative subject.