Shabbat Shekalim is the last Shabbat preceding the beginning of the Hebrew month of Adar*, or the 1st of Adar itself if the month begins on Shabbat. In synagogues, the Torah portion describing the bringing of the silver half-shekel is read (Exodus 30:11-16) in addition to the regular Torah reading, in what is referred to as the maftir portion.
In Temple times, the nation began to give the half-shekel on the first day of the Hebrew month of Adar at collection tables around the country and continued until the beginning of the next month; Nisan. The half-shekel donations were used to support the Jewish Temple, and reading about the mitzvah acted as a gentle reminder for people to give.
A special haftarah, a section taken from the Prophets which is thematically linked to the Torah portion, is also read on Shabbat Shekalim. This special reading is taken from II Kings 11:17-12:17. In this section, Jehoash, the king of Judah, commanded that all money brought to the Temple be used for its repairs and renovations–both the required contributions and the free-will offerings.
The haftarah notes the fact that those who were charged with distributing the money were trusted to such a degree that they did not need to present an accounting of their expenditures, “for they dealt honestly” (II Kings 12:16)
The Israel Bible explains:
According to Jewish law, money that is designated for use in the Beit Hamikdash is endowed with a special status and must be used only for its intended purpose. In the Holy Temple, it is therefore necessary to ensure that donated funds are properly secured and then distributed for care of the Beit Hamikdash and communal offerings. Today, without a Beit Hamikdash, we are likewise required to make sure that we are completely honest in our financial dealings, and that money is used for its intended purpose.
In this haftarah, two covenants were made with the people. The first was a covenant with God: both the king and his subjects would from now on remain faithful to the Torah and its commandments. The second covenant made on that day was between the new king and the people. They swore to be faithful to the king, and he to them.
In addition to reminding the people to donate their half-shekel to the Temple, the portion of Shekalim is also preparation for the holiday of Purim. During Purim, the importance of national unity of Israel is emphasized. Even in the exile in Persia where God did not come to the aid of the Jews in any revealed manner, salvation came when the Jews united against their enemies. This is reflected in the Purim celebrations in which Jews give gifts of food and hold festive meals together. This idea of unity was expressed in the Temple with the donations of the half-shekels. Everyone, young, old, rich, and poor, was required to give a half-shekel donation, and each of these half-shekels were combined to bring the communal offerings. Reading the portion about the half-shekel, which came in lieu of a census of the people, emphasizes that unity. As The Israel Bible explains:
Moshe is commanded to count the Nation of Israel. However, he is not to count individuals. Instead, each person being counted is to make a donation of half a shekel to the Mishkan, and the half shekel coins are to then be counted. Rabbi Samson Raphael Hirsch explains the symbolism of this method: Merely existing among others does not make one an integral part of a society. In order to be counted as part of the nation, each member has to give of themselves and contribute to the community.
*NOTE: The Hebrew calendar is based on a 19-year cycle in which 13 of the years are leap years and have a month added. Leap years have two months of Adar; Adar Aleph and Adar Bet. Purim is celebrated in Adar Bet so Shabbat Shekalim in a leap year precedes Adar Bet.