Lasting four days, Hari Raya Haji commemorates Prophet Ibrahim’s (Abraham's) complete faith and trust in God.
This is recounted in the story of God commanding Ibrahim to sacrifice his son Ismail (Ishmael), a commandment that Ibrahim responded to with obedience. God stopped him, and provided him with a sheep to substitute as a sacrifice, instead of his son.
During this period, the faithful dress in their finest clothes and congregate in mosques to listen to sermons and offer their prayers.
But the most important ritual observed here is that of 'korban' (sacrifice). Worshippers contribute live sheep, lambs, goats and cows, which are slaughtered by a quick slit to the jugular as prayers are recited.
This act reminds worshippers of the willingness of Prophet Ibrahim to offer up even his own flesh and blood to God. The animal is then cleaned and the meat carved up and distributed. (From Singapore Tourism Board website)