The pilgrimage to Makkah (the hajj) is an obligation only for those who are physically and financially able to do so. Nevertheless, over two million people go to Makkah each year from every corner of the globe providing a unique opportunity for those of different nations to meet one another.

The annual hajj begins in the twelfth month of the Islamic year (which is lunar, not solar, so that hajj and Ramada-n fall sometimes in summer, sometimes in winter). Pilgrims wear special clothes: simple garments that strip away distinctions of class and culture, so that all stand equal before God.

The rites of the hajj, which are of Abrahamic origin, include going around the Ka'bah seven times, and going seven times between the hills of Safa and Marwa as did Hagar (Hajir, Abraham's wife) during her search for water. The pilgrims later stand together on the wide plains of 'Arafat (a large expanse of desert outside Makkah) and join in prayer for God's forgiveness, in what is often thought as a preview of the Day of Judgment.
The close of the hajj is marked by a festival, the 'Id al Adha, which is celebrated with prayers and the exchange of gifts in Muslim communities everywhere. This and the 'Id al Fitr, a festive day celebrating the end of Ramada-n, are the two holidays of the Islamic calendar.

"Allah Almighty says, "Hajj to the House is a duty owed to Allah by all mankind – those who can find a way to do it. But if anyone rejects, Allah is Rich beyond need of any being." (3:97)

“Our Lord! And make us submissive unto You and of our offspring a nation submissive unto You, and show us our Manasik (all the ceremonies of pilgrimage - Hajj and Umrah, etc.), and accept our repentance. Truly, You are the One Who accepts repentance, the Most Merciful.” (Quran: 2/128)
Verily! As-Safa and Al-Marwah (two mountains in Makkah) are of the Symbols of Allah. So it is not a sin on him who performs Hajj or Umrah (pilgrimage) of the House (the Kabah at Makkah) to perform the going (Tawaf) between them (As-Safa and Al-Marwah). And whoever does good voluntarily, then verily, Allah is All-Recognizer, All-Knower. (Quran: 2/158)

And perform properly (i.e. all the ceremonies according to the ways of Prophet Muhammad (p.b.u.h.)), the Hajj and Umrah (i.e. the pilgrimage to Makkah) for Allah. (Quran: 2/196)

The Hajj (pilgrimage) is (in) the well-known (lunar year) months (i.e. the 10th month, the 11th month and the first ten days of the 12th month of the Islamic calendar, i.e. two months and ten days). So, whosoever intends to perform Hajj therein (by assuming Ihram), then he should not have sexual relations (with his wife), nor commit sin, nor dispute unjustly during the Hajj. And whatever good you do, (be sure) Allah knows it. And take a provision (with you) for the journey, but the best provision is At-Taqwa (piety, righteousness, etc.). So fear Me, O men of understanding! (Quran: 2/197)

There is no sin on you if you seek the Bounty of your Lord (during pilgrimage by trading, etc.). Then when you leave Arafat, remember Allah (by glorifying His Praises, i.e. prayers and invocations, etc.) at the Mashar-il-Haram. And remember Him (by invoking Allah for all good, etc.) as He has guided you, and verily, you were, before, of those who were astray. (Quran: 2/198)

And Hajj (pilgrimage to Makkah) to the House (Kabah) is a duty that mankind owes to Allah, those who can afford the expenses (for ones conveyance, provision and residence); and whoever disbelieves (i.e. denies Hajj (pilgrimage to Makkah), then he is a disbeliever of Allah), then Allah stands not in need of any of the Alameen (mankind and Jinns). (Quran: 3/97)