During the month between Purim and Passover, there are four Shabbatot with special maftir readings. They are called Shabbat Shekalim, Shabbat Zachor, Shabbat Parah, and Shabbat Hachodesh.
Shabbat Shekalim takes place during the Shabbat prior to Rosh Chodesh Adar, or in a leap year, Adar II. The maftir reading describes a census requiring every Israelite person to contribute a half shekel (a coin used in ancient times) to support the community. Every member of the community’s participation is very important: “the rich shall not pay more, and the poor shall not pay less than half a shekel." The money collected was used throughout the year to purchase the communal sacrifice offered in the Holy Temple in Jerusalem.
Shabbat Zakhor occurs on the Shabbat before Purim. The maftir reading directs us to remember Amalek, the nation that surprised the Israelites wandering in the desert after the Exodus from Egypt with a rear attack. This commandment is fulfilled each year by publicly reading this passage on the Shabbat before Purim, as Haman was a descendant of Agag, King of the Amalekites.
Shabbat Parah, the Sabbath of the Red Heifer, occurs on the Shabbat prior to Shabbat in which we announce the month of Nisan. The maftir reading deals with the red heifer whose ashes were combined with water to ritually purify anyone who had been in contact with a dead person. Because only people who were pure could eat from the Passover sacrifice, a public announcement right before Nisan reminded anyone who had become impure to purify themselves before making the Passover pilgrimage to Jerusalem.
Shabbat HaChodesh occurs either on the Shabbat before Rosh Chodesh Nisan or on Rosh Chodesh itself. The maftir reading details eating the Passover sacrifice, with "your loins girded, your sandals on your feet, and your staff in your hand"; eating bitter herbs and unleavened bread; and putting blood on the doorposts; and it lists the Passover laws.