Charles Ellicott 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)

In the third month. —And so at the end of wheat harvest, the third month (Sivan) corresponding to our May–June. Pentecost, the Feast of Harvest, or Firstfruits, fell in this month.

To lay the foundation. — To found, or lay. Hebrew, lîsôd, a curious form only found here. (Compare to lîsôd,Isaiah 51:16.)

In the seventh month. —Tisri (September to October), in which was held the great Feast of Tabernacles, after all the fruits had been gathered in, and the vintage was over.