I have decided one Old Testament passage I think is definitely The Millennium. The end of Zachariah 14.
And it shall come to pass, that every one that is left of all the nations which came against Jerusalem shall even go up from year to year to worship the King, the LORD of hosts, and to keep the feast of tabernacles. And it shall be, that whoso will not come up of all the families of the earth unto Jerusalem to worship the King, the LORD of hosts, even upon them shall be no rain. And if the family of Egypt go not up, and come not, that have no rain; there shall be the plague, wherewith the LORD will smite the heathen that come not up to keep the feast of tabernacles.
This shall be the punishment of Egypt, and the punishment of all nations that come not up to keep the feast of tabernacles. In that day shall there be upon the bells of the horses, HOLINESS UNTO THE LORD; and the pots in the LORD'S house shall be like the bowls before the altar. Yea, every pot in Jerusalem and in Judah shall be holiness unto the LORD of hosts: and all they that sacrifice shall come and take of them, and seethe therein: and in that day there shall be no more the Canaanite in the house of the LORD of hosts.This theme of punishing Nations I don't see happening in the New Creation. This fits my suspicion that The Millennium is not going to be as Utopic as everyone assumes. The passages I disagree with seeing as The Millennium are about the Gentile Nations enthusiastically worshiping Yahweh.
That only Tabernacles is mentioned here I find interesting. I doubt it'll be the only of the Feast Days kept at this time, but it may be the only one that God will require the Gentiles to come to. Ezekiel 45 has Passover/Unleavened Bread and Tabernacles still being kept, but not First Fruits, Pentecost, Yom Teruah or Yom Kippur.
You may be thinking, "having a required pilgrimage for the Jews is one thing, but do you really think the whole world's population is gonna fit in Jerusalem?" Well what Zechariah 14 says is focused on the Nations, not on individuals. It may be each nation will send representatives there, or that the leaders are supposed to go there.
It is my view that the Fulfillment of Tabernacles is after the Millennium and the Gog and Magog war. While Yom Teruah is fulfilled at The Rapture.