If the Egyptians do not go up and celebrate, they will have no rain. Instead, there will be the plague that Adonai will inflict on the nations that do not go up to celebrate Sukkot.
“And it will come to pass, that from one New Moon to another, and from one Shabbat to another, all flesh will come to bow down before Me,” says Adonai.
Now this is the plague with which Adonai will strike all the peoples that wage war against Jerusalem: their flesh will rot while they are standing on their feet; their eyes will rot in their sockets; and their tongues will rot in their mouths.