On the 1st of Adar, the announcement to prepare the Shekalim was made: Megillah 13b, 29a-b
On the 15th of Adar, special money-changers were established to aid acquisition of the Half-Shekel for its donation: Megillah 29b
On the 25th of Adar, special money-changers were established in the Temple itself to aid acquisition of the Half-Shekel for its donation: Megillah 29b
Donation of the Shekalim
If an individual desires to donate more than his half-shekel, he must fully give over his money to the community before offering it: Rosh HaShanah 7a-b