لِيَوْمٍ عَظِيمٍۢ
Sentence structure
لِيَوْمٍ is linked to مَّبْعُوثُونَ in the preceding ayah, and عَظِيمٍ is its genitive adjective.
Ayah summary
They will have to give account before Allah on a tremendous day.
لِيَوْمٍ — For a Day
Grammar: لِ is a preposition and يَوْمٍ its genitive noun; the phrase is linked across the ayah boundary to مَّبْعُوثُونَ.
عَظِيمٍۢ — Great
Grammar: An indefinite genitive adjective describing يَوْمٍ.