Rejoicing can be commanded because it begins with what the hands do, not with what a person happens to feel.
Gratitude that waits on the mood turns up about twice a year. Arms full of palm and willow change the breathing, the breathing changes the state, and the state produces the very joy Yahweh asked for. He prescribed the physiology first, knowing which end of a person is easier to move.
Seven days is the other half of the design. A state entered once is an experience; a state entered daily for a week trains the body to find that joy faster the next time it is called for, and whatever a person reaches quickly becomes where he mostly lives. A household that skips the branches leaves its worship at the mercy of whatever happened that morning.