Shekalim 2 - On the first of Adar
Many things are done by the community on various dates in the month of Adar . Thus, on the first of Adar the Sanhedrin used to announce that everybody must prepare to pay the half-shekel. Every Jew would give half a shekel yearly; the money was stored in a Temple’s chamber, and the priest would use it to buy sacrifices. These were the daily sacrifices used to atone for everybody, and that is why one everyone had to take part in donating money for them.
Why was this announced on the first of Adar? - Because on the first of the next month, Nissan, the priests would go and new money for the Temple’s needs. Since each year needed "fresh" new shekalim, and the remainder of the last’s years coins were used for other needs, as we will see, everybody was given thirty days to prepare the payment.
Art: The Tibute Money by (after) Sir Peter Paul Rubens