On this day, the bottom of Noah's ark, submerged 11 cubits beneath the water's surface, touched down and came to rest on the top of Mount Ararat.
(This follows the opinion of the Talmudic sage Rabbi Joshua, who maintains that the Flood began on Iyar 17.)
See: Great Flood Begins.
"Pulver Purim" was established by Rabbi Avraham Danzig (1748-1820) author of the halachic works Chayei Adam and Chochmat Adam, after he and his family were miraculously saved from a fire on the eve of the 16th of Kislev.
The inferno engulfed many homes, including his own home and the very room where all of his family members were, causing some of the walls to collapse. Rabbi Avraham Danzig then established the 16th of Kislev as a day of celebration for all of his future descendents.
On Yom Kippur, the souls return to connect to their origin, carrying with them the divine sparks from all they've done in this world.
Their return rises like an aroma of burning incense through the atmosphere, and through endless strata of spiritual realms, until it reaches a place beyond higher and lower. It reaches the essence of G-d.
And there it becomes the essence of pure and ultimate bliss.
On Sukkot, that divine, supernal delight reappears in our world, much as a cloud condenses from the vapor rising from the sea.
And it manifests as the branches and leaves covering our Sukkah.
So that within the Sukkah, we are embraced by G-d’s delight in us, and we rejoice.