• On Taanit Esther – the “fast of Esther,” before the Mincha prayer, we give the Machatzit HaShekel. A Machatzit HaShekel is half of the standard currency of that particular country.
• It is customary to give three coins, since the word Terumah (lit. an offering) is mentioned three times in the beginning of Parshat Ki Tisa (Exodus 30:11-34:35). If one did not manage to give it before Mincha, then he can give it after Mincha, or before the Megillah reading on Purim night or before the Megillah reading on Purim morning.
• One should not use money from his Maaser (tithes) for Machatzit Hashekel.
• A father who started giving on behalf of his young children, must continue giving on their behalf every year until the child becomes responsible to give on his own, unless he explicitly made a condition [the first time that he gave for his children] that he is not accepting it upon himself as a vow. A father should also give the Machatzit Hashekel on behalf of his unborn child (during the mother’s pregnancy)
– Vadim Segal