Four "special Sabbaths" occur just before the start of spring: two before the holiday of Purim and two before the holiday of Passover. Collectively, these Sabbaths are called "The Four Shabbatot" and four additional Torah readings (called Arba Parashiyot, or the "four portions") are read on each of these Sabbaths in preparation for the holidays, particularly for the central holiday of Passover.
The first of the four Sabbaths is called Shabbat Shekalim (שבת שקלים), "the Sabbath of the Shekels," which occurs just before the month of Adar begins.
An additional reading (Exod. 30:11-16) is appended to the regular Torah reading that describes the contribution of a half-shekel for the construction and upkeep of the Mishkan (Tabernacle).