The Seventeenth of Tammuz was supposed to be the day on which the Tablets of the Covenant were received and given to Israel, and the Ninth of Av was the day when the Spies would to return from the Land of Israel and proclaim, “The land is very good!”
The Breaking of the Tablets on the Seventeenth of Tammuz
The Tree of Knowledge contained a mixture of elements - both good and evil. This, then, was Adam's sin: He ate from the tree, and this caused existence to fall to a level whereupon there existed a mixture of both good and evil.