(For the sake of my friends who dont understand the difference btw Hari Raya Haji -aka Hari Raya Aidil Adha- and Hari Raya AidilFitri)

Muslims all over the world observe Eid-ul Adha (in Arabic) or Hari Raya Aidil Adha (in Malay) on the 10th day of Zulhijah, the last month of the Muslim calendar. This festival is celebrated in conjunction with the pilgrimages performed by Muslims, called the Haj and to commemorate Prophet Abraham's spirit of Qurban or sacrifice. Thus, in Singapore, Hari Raya Aidil Adha is also known as Hari Raya Haji or Hari Raya Qurban.
The Haj is a pilgrimage to the holy places in and around the holy city of Mecca (the birthplace of Islam) in Saudi Arabia where pilgrims will perform certain religious rituals and prayers. It is one of the central religious duties of Muslims and is enshrined in the Quran. According to the fifth tenet of Islam, Muslims who are financially and physically able are required to perform a pilgrimage to the holy city of Mecca at least once in their lifetime. The pilgrimage is also inspired by the earlier example of obedience to God set by the Prophet of Abraham. Pilgrims wear special clothes: simple garments which strip away distinctions of class and culture, so that all stand equal before God.
After the completion of the Haj, the pilgrims would perform the 'qurban' or sacrifice. This act is in remembrance of Prophet Abraham's willingness to sacrifice his elder son, Ishmael, when God commanded him to do it as a test of his commitment to his faith. At the last moment, God miraculously replaced a ram in place of Ishmael.
In Singapore, the day is spent offering thanksgiving prayers and prayers for forgiveness at the mosques early in the morning. After prayers, Muslims conduct the 'qurban' - slaughtering of animals such as lambs and goats. Though not a compulsory religious duty, it is considered an obligation for those who can afford. The meat is distributed to neighbours, friends and most importantly to the poor and needy while keeping a portion (usually 1/3) for their family. It is customary for Muslims to visit their parents and relatives where a wide selection of traditional Malay food is served.
Hari Raya Aidiladha is different from Hari Raya Puasa, also known as Aidil Fitri. Hari Raya Puasa, which falls on the 1st of Syawal, the tenth month of the Islamic calendar, marks the end of Ramadan, the month of fasting. It is a day of victory for Muslims, having successfully completed the fast.

(source: The Net)