Albert Barnes Commentary 2 Chronicles 31:7

"In the third month they began to lay the foundation of the heaps, and finished them in the seventh month." — 2 Chronicles 31:7 (ASV)

The third month. Compare 2 Chronicles 29:3; 30:2, 13. The events previously described—the destruction of the high places, the reappointment of the priestly courses, and the reestablishment of the tithes—followed the Passover so closely that the gifts began to pour in before a month had passed since the feast’s conclusion. In the seventh month, the harvest was completed, and the last tithes and firstfruits of the year would naturally have come in then.