﴿ يَوْمَ تَكُونُ ٱلسَّمَآءُ كَٱلْمُهْلِ ﴾

“[It will take place] on a Day when the sky will be like molten lead,”

The day when the heaven will be yawma takūnu’l-samā’u is semantically connected to an omitted clause implicitly taken to be yaqa‘u ‘it will take place’ as molten silver