Jamieson, Fausset and Brown on Leviticus 26:5

5. your threshing shall reach unto the vintage, and the vintage shall reach unto the sowing time, &c.--The barley harvest in Judea was about the middle of April; the wheat harvest about six weeks after, or in the beginning of June. After the harvest came the vintage, and fruit gathering towards the latter end of July. Moses led the Hebrews to believe that, provided they were faithful to God, there would be no idle time between the harvest and vintage, so great would be the increase. (See Am 9:13). This promise would be very animating to a people who had come from a country where, for three months, they were pent up without being able to walk abroad because the fields were under water.