Midrasch zu Shekalim 3:5

When thou takest the sum of the children of Israel (Exod. 30:12). May our masters teach us: How many times each year did the Israelites bring their offerings to the Temple? Thus did our masters teach us: They brought them three times a year; on the first day of the months of Nisan, Iyar, and Elul.¹Our text says “Adar, Nisan, and Elul.” Etz Joseph, however, suggests that the reading should be “Nisan, Iyar, and Elul.” Cf. M. Shekalim 3:1. On the first day of the months of Nisan and Iyar the offerings for the Temple treasury would be collected and the priests would approach the altar to seek forgiveness for the sins of Israel with the shekels they had contributed. But why did they do so three times a year? In order that all the Israelites might be involved, throughout the year, in giving their contributions. Why did they begin to accumulate their contributions on the first day of Adar,²Actually, the Sanhedrin would make the announcement on the first day of Adar. though they did not bring it in until the first day of Nisan? It was done that way so that the offering would not become an unbearable hardship for the Israelites. Hence they (the priests) would remind the Israelites on the first day of the month of Adar (to prepare their offerings).