The standing at Arafat (wuquf) is the greatest pillar of Hajj, as the Prophet (peace be upon him) said: 'Hajj is Arafat.' Its time begins after zawal (when the sun passes its zenith) on the 9th of Dhul-Hijjah according to the majority of scholars, and extends until the dawn (fajr) of the 10th of Dhul-Hijjah. A pilgrim who is present at Arafat for even a moment during this time window has fulfilled the pillar of Hajj.
Sheikh Ibn Baz explained that the Sunnah is to arrive at Arafat before zawal, pray Dhuhr and Asr combined and shortened, and remain there supplicating until sunset. However, a pilgrim who arrives late — even in the last moments before fajr of the 10th — has caught Arafat and their Hajj is valid. The Prophet said: 'Whoever comes to Arafat before dawn on the night of Jam' (Muzdalifah), his Hajj 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 pilgrim who only stands at Arafat at night misses the Sunnah of daytime standing and the great merit of supplication on the afternoon of Arafat. He recommended every effort to arrive during the daytime. If a pilgrim 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.