Giving Machatzis HaShekel
It is the custom to give (just) before Purim, half of the standard monetary unit in use in one's local place and at the current time. in memory of the half-shekel that was given in (every) Adar, in order to buy the (animals for the) public sacrifices. It's customary to give three (times the) half (monetary) units because in Parashat Ki Tisa,1 it writes the word ''Terumah'' three times.2 One gives (this money) in the evening, before the Megillah reading, and it is distributed to the poor. A minor is exempt (from this donation), but if his father gave on his behalf one time, (he is) obliged (to continue to give) always. (Concerning) a thirteen year old boy, some (authorities) say (that) he is obligated (to donate), and some (others) say that he is exempt until he is twenty. (KSA: 141:5)
1) Ex. 30:11.
2) ''Terumah'' means a donation. From Rosh Chodesh Nissan onwards, the animals used for all the communal sacrifices in the Temple, had to be purchased using money collected that year. Therefore, one month before, on Rosh Hodesh Adar, announcements would be made to remind people to donate the half-shekel, so that animals could be purchased with the ''new'' money before Nissan. The reading of Parashat Shekalim just before (or on) Rosh Hodesh Adar also serves as a reminder of that mitzvah.