In wa-yawma, the conjunction links this episode to the preceding reminiscence, while yawma functions as a temporal adverbial. Dakhaltu is a perfect verb with the first-person subject suffix -tu.
Al-khidra is accusative after dakhaltu. The following phrase, khidra ʿUnayzah, identifies the same enclosure and can be parsed as an appositional substitution. ʿUnayzah completes the construct and, as a diptote proper name, bears fatḥah in the genitive position.
Taken together, the verse identifies the occasion of entry and immediately follows it with the woman’s reaction, introduced by fa-qālat.