In the Millennial kingdom the Festival of Tabernacles, the seventh and final feast of Israel, will be celebrated yearly for the entire duration of the Millennium, and be held in Jerusalem.  All nations, meaning everyone on the Earth, will come down to Israel and celebrate the feast yearly.  This will not be an optional commandment to obey.  Everyone, and that literally means EVERYONE, down to the last man, woman and child, by requirement of God MUST be there.  I'm not sure what the punishment is if you don't go, but the law says that all must be there.

But it's about more than just a commandment to appear in Jerusalem before Christ.  It's a symbol of restoration.  This will be a time of fellowship, restoration and union with Jesus Christ.