Numbers 13:25

And they returned from searching the land after forty days. The Targum of Jonathan adds, on the eighth day of the month Ab, which answers to part of July and part of August; so that this must be towards the latter end of July: some Jewish writers k say it was the ninth of Ab; hence the tradition, that it was decreed on the ninth of Ab concerning their fathers, that they should not enter into the land l.

k Seder Olam Rabba, c. 8. p. 24. l Misn. Taanith, c. 4. sect. 7.