The pilgrimage to Mecca (the Hajj) has, as it's most important
day of the five to eight day event, the "standing" at Arafah. The
Mount of Mercy, as it is called, is a hill within the large valley
called Arafah.
It is not essential to stand on the Mount of Mercy, though many
people do because Muhammad did. What is important is to spend the
entire day within the area called Arafah. On that day - the ninth
day of the twelfth month (called Thul-Hijjah, or "the month of the
Hajj") God causes His Mercy to descend upon all who are gathered
there sincerely for the worship of Him Alone.
What happened in the time of Muhammad (peace be upon him) on the
Mount of Mercy was that, during the pilgrimage, Muhammad stood on
that hill and addressed the people. In that address in that
pilgrimage (which is called the "farewell Pilgrimage" due to the
fact that Muhammad died before the next year) the Prophet of God
expressed many important things to his nation. he also hinted that
he would not be alive the coming year to make the pilgrimage
again.
If you are interested, you can Google the "Farewell Pilgrimage"
and read a copy of the sermon he gave.
