Albert Barnes Commentary Numbers 13:20

"and what the land is, whether it is fat or lean, whether there is wood therein, or not. And be ye of good courage, and bring of the fruit of the land. Now the time was the time of the first-ripe grapes." — Numbers 13:20 (ASV)

The time ... of the firstripe grapes - The first grapes ripen in Palestine in July and August: the vintage is gathered in September and October. This indication of date agrees with what we would have inferred from the previous narrative.

For the Israelite host had left Sinai on the 20th day of the second month (Numbers 13:10, Numbers 13:11), or about the middle of May. Since then, they had spent a month at Kibroth-hattaavah and a week at Hazeroth, and had accomplished, in all, from 150 to 200 miles of march. Therefore, it must have been at least the beginning of July, and may have been a month later, when the spies were sent into the land of promise.