Yawma is annexed to idh. The clause that idh would normally govern is omitted; the tanwīn on idh compensates for that omitted clause.
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Structure of Yawmaʾidhin
Tanwīn: Five Types, Their Function, and Omission
Yawma is annexed to idh. The clause that idh would normally govern is omitted; the tanwīn on idh compensates for that omitted clause.