To quote the Psalms (126:2): “Then it was said among the nations, The L-rd has done great things for them.” And this is how our commentators interpret it: “Then it was said among the nations” — when Moshiach comes, the nations of the world will say — “The L-rd has done great things for them.” The nations will be rubbing their eyes inquire in unconcealed disbelief — “Why has Almighty wrought the miracles of the Redemption only for Jews?”
And how will we respond? — “A great thing G-d has performed for us” — specifically for us and for no one else! And why is that? On the account of our being joyful. Everything will be done in the merit of our celebration. If that’s so, let’s start celebrating already now and thus bring the revelation of Moshiach closer!