The standing at عرفة (wuquf) is the greatest pillar of الحج, as النبي (صلى الله عليه وسلم) said: 'الحج is عرفة.' Its time begins after zawal (when the sun passes its zenith) on the 9th of Dhul-Hijjah عند جمهور العلماء, and extends until the dawn (fajr) of the 10th of Dhul-Hijjah. A الحاج who is present at عرفة for even a moment during this time window has fulfilled the pillar of الحج.
الشيخ ابن باز explained that the السنة is to arrive at عرفة before zawal, pray Dhuhr and Asr combined and shortened, and remain there supplicating until sunset. However, a الحاج who arrives late — even in the last moments before fajr of the 10th — has caught عرفة and their الحج is valid. النبي said: 'Whoever comes to عرفة before dawn on the night of Jam' (مزدلفة), his الحج is complete.' This hadith establishes that the nighttime of the 9th is also valid time for wuquf.
الشيخ ابن عثيمين noted that while arriving late at night is valid, the الحاج who only stands at عرفة at night misses the السنة of daytime standing and the great merit of الدعاء on the afternoon of عرفة. He مستحب every effort to arrive during the daytime. If a الحاج arrives after sunset but before fajr, their standing is valid but they should offer a dam (sacrifice) عند 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.