--Gemara Megillah 29a, SA 685:1
On the Shabbos before Rosh Chodesh Adar, we take out two Sifrei Torah and read Parshas Shekalim from the second Torah as the Maftir for that week. During the times of the Beis HaMikdash, the announcement would be made in every Beis Din regarding the required donations on the first of Adar. It was time for every Jew to give a half shekel to the Beis HaMikdash to purchase the public Korbonos from the newest money when Nisan began.
--Nitei Gavriel