it is a sunnah to leave for arafah after the sunrise of the 9th day of dhul-hijjah. this is while uttering takbir, which is saying: allahu akbar (god is the greatest), tahlil, which is saying: la ilaha ila-allah (there is no god but allah) and talbiyah, which is saying: labbayka allahumma labbayk.
the importance of this stand is related to the amount of forgiveness and mercy bestowed by god on his pilgrims. the prophet (peace be upon him) spent the day at arafah until almost sunset. then he said:
"o bilal! ask the people to be quiet and listen to me."
bilal stood up and asked the people to be quiet and listen to the prophet. when they were quiet, the prophet said:
"o people! a little while ago gabriel (peace be upon him) came to me, gave me salutations from allah, and informed me that allah has forgiven those who spend the day at arafah…"
at this, umar ibn al-khattab stood up and asked:
"o allah's messenger, is this for us only?"
the prophet said:
"this is for you, and for all those who will come after you, until the day of judgment…"
"how plentiful and blissful allah's bounties are!" (at-targhib wat-tarhib and authenticated by al-albani)
the prophet also said:
"allah frees far more people from hellfire on the day of arafah than on any other day. allah comes closer this day and proudly says to the angels: ‘what do these people want and seek?!'" (muslim)
in fact, the importance of standing on the mountain of arafah during the journey of hajj is highly confirmed in the prophet's very words:
"hajj is arafah." (an-nasa'i)
most scholars are of the opinion that the time to spend in arafah begins from noon of the 9th day of dhul-hijjah until dawn of the 10th of dhul-hijjah. they mostly agree that any part of this period of time, day or night, may be spent in arafah to fulfill its condition, as being an anchor of hajj.
staying at arafah means physical, as well as mental presence in any part of arafah. this is whether one is awake, asleep, riding, sitting, lying down or even walking…
it is recommended to seek forgiveness of allah as much as possible, glorify allah, and supplicate for one's well-being and welfare in this life and in the hereafter. it is also recommended to supplicate for the welfare of others. this is to be done sincerely with fervor, attention and with hands raised humbly in supplication.
as for the recommended supplications, it was reported that on the day of arafah the prophet (peace be upon him) would say:
"there is no deity worthy of worship but allah alone. he has no partners. to him alone, belongs the kingdom, and all praise. in his hand is all the good, and he has power over all things."
again, it was reported that prophet muhammad (peace be upon him) said:
"the supplication most frequently used by the prophets before me and by me on the day of arafah is this:
‘there is no deity worthy of worship but allah alone. he has no partners. to him belongs the kingdom and all praise, and he has power over all things. o allah! place light [guidance] in my sight, in my ears and in my heart. o allah! open my heart and ease my tasks. o allah! i seek refuge with you from evil whispers, from confusion, from the trial of the grave, from the evil that takes place during the day or night, from the evil carried by the winds, and from the afflictions of the world.'"
according to sunnah, pilgrims should leave arafah quietly and peacefully after sunset. the prophet left arafah very peacefully and quietly, while saying:
"o people, walk calmly; rushing or making hast is not a virtue…" (al-bukhari and muslim)
the inner peace to be felt then is a reflection of the pilgrim's certainty that he/she has been accepted. this in itself is a sign of faith in allah, because it is his promise to accept those whom are granted that blessed stand.
hope this answer is satisfactory, tarek. may allah grant you, and all of us, a blessed stand on the mount of forgiveness next year in sha' allah!
brother mohsen haredy, a member of ask about islam editorial staff, would like to add the following:
it is highly recommended for non-pilgrims to observe fasting on the day of arafah. the prophet (peace be upon him) exhorted us to do good deeds during the first ten days of dhul-hijja and fasting is one of those good deeds. abu qatada al-ansari (may allah be pleased with him) reported that: "…. he (the prophet) was asked about fasting on the day of arafah, whereupon he said: it expiates the sins of the preceding year and the coming year." (muslim)
as for pilgrims, they are not allowed to fast on the day of arafah so that they will be strong enough to worship and supplicate allah on this great day. abu hurairah (may allah be pleased with him) reported that the prophet "forbade fasting on the day of arafah for one who is actually at arafah." (ahmad)
the prophet himself did not fast the day of arafah when he performed hajj. umm al-fadl bint al-harith reported that some people argued about the fasting of the messenger of allah on the day of arafah. some of them said that he had been fasting, whereas the others said that he had not been fasting. i sent a cup of milk to him while he was riding his camel at arafah, and he drank it. (muslim)