And it shall come to pass that everyone who is left of all the nations which came against Jerusalem shall go up from year to year to worship the King, the LORD of hosts, and to keep the Feast of Tabernacles.
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Verses 17-19 describe the punishment on those nations that choose to disobey and not keep the Feast of Tabernacles:
“And it shall be that whichever of the families of the earth do not come up to Jerusalem to worship the King, the LORD of hosts, on them there will be no rain. If the family of Egypt will not come up and enter in, they shall have no rain; they shall receive the plague with which the LORD strikes the nations who do not come up to keep the Feast of Tabernacles. This shall be the punishment of Egypt and the punishment of all the nations that do not come up to keep the Feast of Tabernacles.”
The Feast of Tabernacles was a commanded festival in the Old Testament; Jesus Christ and the apostles and the New Testament Church continued to observe it. And it will be required after Jesus Christ returns to establish the Kingdom of God.