989. Machatzis HaShekel

141:5 The practice before Purim is to donate half of the monetary unit of one's time and place (e.g., half a dollar, half a pound, etc.) in memory of the half-shekel that was donated in Adar to purchase animals for the communal sacrifices. The custom is to give three times this half unit (e.g., $1.50) because parshas Ki Sisa uses the word "terumah" (donation) three times. (It is not clear whether someone who survives on charity funds is obligated to donate - Bi'ur Halacha 694:1 s.v. Yeish omrim.)

This money is donated in the evening, before reading the Megillah, and is distributed to the poor. (Some do so in the morning - Mishnah Brurah 694:4.) A minor need not donate but once his father gives on his behalf, he is obligated to continue doing so. Some authorities say that a thirteen year old boy is obligated to donate, while others say that a person is exempt until the age of twenty. (The custom is to give on behalf of all members of one's family, even those still in utero - MB 694:5.)

141:6 On Purim - which is the 14th day of Adar - we recite "al hanissim" in Shemoneh Esrei. If one forgot, the rules are the same as on Chanukah. (See above, 139:21.)