Erev Purim, 13
Adar, is a fast day known as the "Fast of Esther" (see p. 108 for the significance of this name).
The fast begins at alot hashachar — the break of dawn — and one must fast till after Ma'ariv and the reading of the Megillah is completed.
Before Minchah it is customary to commemorate the half-shekel that was given during the month of Adar when the Beit Hamikdash was standing. Since the word "terumah" — "offering" — is mentioned three times in the Biblical portion (Shemot 30:11-16) which discusses the half-shekel, we give three coins which are half the local currency (e.g. three half-dollars in the U.S.).
The half-shekel should not be from one's ma'aseir funds.
There are various opinions as to at what age the obligation begins to give a half-shekel. The Chabad custom is to give for every member of the family regardless of age.
When Purim falls on Sunday, the fast and the giving of the half-shekel takes place on the preceding Thursday.
