The book calls this compensation for the mudāf ilayh. More exactly in yawmaʾidhin, the clause following idh has been omitted, not the word yawma.
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The Missing Complement Is a Clause
Tanwīn: Five Types, Their Function, and Omission
The book calls this compensation for the mudāf ilayh. More exactly in yawmaʾidhin, the clause following idh has been omitted, not the word yawma.