The reason for it is given in vv. 4, 5 _a_. It--_i.e._, the feast (not the musical accompaniments)--is appointed by God. The psalmist employs designations for it, which are usually applied to "the word of the Lord"; statute, ordinance, testimony, being all found in Psalms xix., cxix., with that meaning. A triple designation of the people corresponds with these triple names for the feast. _Israel_, _Jacob_, and _Joseph_ are synonyms, the use of the last of these having probably the same force here as in the preceding psalm--namely, to express the singer's longing for the restoration of the shattered unity of the nation. The summons to the feast is based, not only on Divine appointment, but also on Divine purpose in that appointment. It was "a testimony," a rite commemorative of a historical fact, and therefore an evidence of it to future times. There is no better proof of such a fact than a celebration of it, which originates contemporaneously and continues through generations. The feast in question was thus simultaneous with the event commemorated, as ver. 5 _b_ tells. It was God, not Israel, as is often erroneously supposed, who "went forth." For the following preposition is not "from," which might refer to the national departure, but "over" or "against," which cannot have such a reference, since Israel did not, in any sense, go "over" or "against" the land. God's triumphant forth-putting of power over the whole land, especially in the death of the first-born, on the night of the Passover, is meant to be remembered for ever, and is at once the fact commemorated by the feast, and a reason for obeying His appointment of it.