Praise be to Allah.
The day of ‘Arafah and the fast thereon is the ninth day of the month of Dhu’l-Hijjah, which is defined for each country according to their sighting of the new moon of Dhu’l-Hijjah. So it may be on Thursday for the people of Makkah, and for others it may be on Wednesday or Saturday. It is not essential to follow the people of Makkah when there are differences in the sighting of the new moon. This is the most correct of the scholarly views, that each country has its own sighting when there is a difference concerning that.
If the Muslims in Britain have sighted the new moon, then the Muslims in that country should follow their sighting, otherwise they should follow the sighting of the country closest to them. See the answer to question no. 40720.
There is a great deal of virtue in offering supplication (du‘aa’) on the day of ‘Arafah, because of the hadeeth of ‘Abd-Allah ibn ‘Amr ibn al-‘Aas (may Allah be pleased with him), according to which the Prophet (blessings and peace of Allah be upon him) said: “The best of supplication is supplication of the day of Arafat, and the best of what I and the Prophets before me said is: Laa ilaaha illa Allah wahdahu, laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer (There is no god but Allah alone, with no partner or associate; His is the Dominion, to Him be praise, and He has power over all things).”
Narrated by al-Tirmidhi (3585) and classed as hasan by al-Albaani in Saheeh al-Targheeb, 1536.
Does this virtue apply only to those who are in ‘Arafat, or does it include other places too?
Concerning that there is a difference of opinion among the scholars, which has been discussed previously in the answer to question no. 70282
According to the opinion that it includes other countries and places too, the same may be said concerning it as is said above. So each person should offer supplication on the ninth day of Dhu’l-Hijjah, according to the sighting of the new moon in his country, even if the pilgrims stood in ‘Arafat on the previous day or will do so on the following day.
And Allah knows best.