The Pentateuch is divided into 54 weekly chapters, and each Saturday morning, a weekly chapter is read in synagogue.
Four times a year, in addition to the weekly chapters, four special Torah passages are read:
Parshat Shekalim - this passage is read on the Saturday before the month of Adar. This chapter describes the law obliging every man to annually allocate half a shekel (in Hebrew, machatzit ha-shekel) to the Temple. The month of Adar marked the beginning of the next machatzit ha-shekel collection. At that time, a shekel was equivalent to approximately 20 grams of pure silver.
Parshat Zachor is read on the Saturday before the holiday of Purim. This chapter contains the commandment to remember ("zachor" - "remember" in Hebrew) what Amalek did to the Jewish people during the Exodus from Egypt. The people of Amalek and the wicked Haman pursued the same goal: the complete annihilation of the Jews. Parashat Para is read on the Saturday after Purim to remind people to ensure ritual purity before Passover (before entering the Temple).
Parashat HaChodesh is read on the last Saturday of the month of Adar and also serves as a reminder of Passover because it describes the laws of the Passover sacrifice.