Psalms 118 is one of the Psalms that is referred to as the "Hallel Psalms". The word "Hallel" simply means "To Praise." This Psalm, along with others from 113-118, were sung at Passover, which the Jewish people reflected back to the Lord's deliverance of the children of Israel from bandage in Egypt. This week we celebrate Palm Sunday from this Psalm and from John 12.
