A: Sukkoth is the Feast associated with prayers for rain, which is what the prophet declares will be denied those nations that refuse to observe it.Rain in turn is the symbol of the gift of Life granted directly from Heaven, and especially the water of Life that Yeshua offered at Hoshannah Rabbah so long ago in Jerusalem. (John 7:37. Note that Zechariah 14:8 also mentions the fountain of living water). As I see it, Sukkoth is also a time for rejoicing in one’s dependence on God. After all, Sukkoth is celebrated by living in an “inadequate” shelter, largely exposed to the sky and symbolically baring oneself to God. I don't mean to imply that the survivors of the judgments spoken of in Zechariah will not literally celebrate the Feast, but I suggest that the reason it is so important is that those nations which refuse to humble themselves before God and acknowledge their utter dependence on him cannot expect his blessing. And by “blessing” I mean the blessing of life. Once God has revealed himself so clearly, there is no excuse for disbelief. Even the sacrifice of Yeshua is useless to those who refuse to accept it. It is not the keeping of the Feast that will save these people, but the reliance on the Savior they will be expressing by doing so. (also note that rabbinic Judaism has made Sukkoth much more complicated than the Torah itself does.)