The standing at Arafah (wuquf) is the greatest pillar of Haji, as Nabi (shallallahu alaihi wa sallam) said: 'Haji is Arafah.' Its time begins after zawal (when the sun passes its zenith) on the 9th of Dhul-Hijjah menurut jumhur ulama, and extends until the dawn (fajr) of the 10th of Dhul-Hijjah. A jamaah haji who is present at Arafah for even a moment during this time window has fulfilled the pillar of Haji.
Sheikh Ibn Baz explained that the Sunnah is to arrive at Arafah before zawal, pray Dhuhr and Asr combined and shortened, and remain there supplicating until sunset. However, a jamaah haji who arrives late — even in the last moments before fajr of the 10th — has caught Arafah and their Haji is valid. Nabi said: 'Whoever comes to Arafah before dawn on the night of Jam' (Muzdalifah), his Haji is complete.' This hadith establishes that the nighttime of the 9th is also valid time for wuquf.
Sheikh Ibn Uthaymeen noted that while arriving late at night is valid, the jamaah haji who only stands at Arafah at night misses the Sunnah of daytime standing and the great merit of doa on the afternoon of Arafah. He sunnah every effort to arrive during the daytime. If a jamaah haji arrives after sunset but before fajr, their standing is valid but they should offer a dam (sacrifice) according to some scholars, as they missed the daytime standing. However, the majority hold that no dam is required as long as any part of the wuquf window was caught.