Today is the last day of the Feast of Tabernacles. It is the eighth day of a seven day festival. I'm serious. See Leviticus 23:36 and Numbers 29:35.
In the Worldwide Church of God tradition in which I was raised, the day was about God giving people a second chance to be saved. According to the WCG, after the millennium (which the Feast of Tabernacles represents), God will resurrect people in the second resurrection. At that time, according to the WCG, God will give people who had no chance to know him in their lifetimes an opportunity to learn of his ways. The WCG interpreted the Feast of Tabernacles in light of the millennium, so it connected the Last Great Day with the next stage of the story: the second resurrection. How it got a second chance from Revelation 20:11-12, however, I do not know.
David Antion, a minister in the WCG tradition, offered an alternative explanation. He argued that the Last Great Day represents the new heaven and the new earth. For Antion, the millennium was a time of transition from the old, sinful world to a new age of righteousness, peace, and joy. Because Antion equated the Feast of Tabernacles with the millennium, he interpreted the following festival in light of what he saw as the next step of the story: the new heavens and the new earth.
All of this is speculative. The Torah really does not tell is what the Last Great Day represents. But, if Christians want to get something out of the day, they do have a New Testament reference. John 7:37-39 says the following:
"In the last day, that great day of the feast, Jesus stood and cried, saying, If any man thirst, let him come unto me, and drink. He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water. (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given; because that Jesus was not yet glorified.)"
In that passage, Jesus most likely refers to a Jewish tradition, which appears in Mishnah Sukkah 4:9 and partially in Josephus' Antiquities 13.13.5. According to the tradition, a priest would pour a water libation at the Water Gate of the Temple. There are various Jewish interpretations of this practice. The Tosefta (third century C.E.), in Sukkah 3:9, interprets the Water Gate in reference to Zechariah 13:1 and Ezekiel 47:9. Both passages refer to a stream that will flow from the Temple. Zechariah 13:1 says that the stream will cleanse Jerusalem of sin and impurity, and Ezekiel 47:9 refers to seawater becoming fresh as a result of the stream.
The Babylonian Talmud, in Rosh Hoshanah 16a, says, "Why did the Torah enjoin on us to pour out water on Tabernacles? The Holy One, blessed be He, said, Pour out water before Me on Tabernacles, so that your rains this year may be blessed." The association of the Feast of Tabernacles with rain goes back to biblical times. Zechariah 14:16-17 says that the nations that do not observe the Feast of Tabernacles after Israel's restoration will receive no rain. The Feast of Tabernacles is designed to bring rain, so the nations that do not keep it will go dry.
Within Judaism, there is a recognition of the physical and spiritual significance of water. On a physical level, humans need water. Scientists have told us that we can survive longer without food than without water. We get thirsty. And water is necessary to create food, which is why the Jews prayed for rain.
But water also has spiritual significance. It cleanses and purifies, which is why we bathe in it. And, according to Jesus, it satisfies. On some level, humans have spiritual thirst that must be quenched for them to live. If it is not satisfied, then they will die inside. How? In Romans 8:6, Paul says, "For to be carnally-minded is death; but to be spiritually-minded is life and peace." Those who are spiritually-minded have peace with God, themselves, and others. And that makes sense. What are the characteristics of the carnally-minded? Hatred of God and others, jealousy, strife, etc. (Galatians 5). These do not describe a mind at rest. But what characterizes the spiritually-minded? Peace with God and others, love, joy, etc. Do you see how the Spirit gives us life and peace? We thirst for peace and rest, and we get them through the Spirit of God, the life-giving water.
As I've said before, I identify more with the flesh than the Spirit. But I am thirsty for what Jesus has to offer, and all I can do is go to him. As the Jews recognized their dependence on God for rain, I see that I need God for rest, peace, and fruit. I am a body of seawater that wants to become fresh.