سَعِدَتْ بِغُرَّةِ وَجْهِكَ الْأَيَّامُ
The days were made fortunate by the brightness of your face.
The grammatical subject «al-ayyām» appears after the verb and the phrase «bi-ghurrati wajhika». The brightness of the addressee’s face is made prominent before the statement of the days’ good fortune is fully completed.
The effect comes from the progression of the verse: praise, anticipation, and completion of attribution. First parse the verb, complement, and subject correctly, then identify the rhetorical benefit of their order.