What is the basic syntactic role of yawma in 101:4?

Supplementary Practice 26 — Comparative Rhetoric of Sūrat al-Qāriʿah and Sūrat al-Tīn

• A temporal adverb framing when the scene occurs
• A nominative subject requiring no predicate
• An oath particle governing a genitive noun
• The fāʾ responding to a prohibition