John Gill 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 they began to lay the foundation of the heaps . &c.] The month Sivan, as the Targum, in which month was the feast of Pentecost, called the feast of the harvest, (Exodus 23:16) , for then barley harvest was ended, and wheat harvest began, and the firstfruits were brought:

and finished them in the seventh month ; the month Tisri, as the Targum, in which was the feast of tabernacles, sometimes called the feast of ingathering, of the fruits of the earth, the wine, oil and so a proper time for bringing the tithes of all.