Machazit Ha'shekel

Resh Lakish said "On the first of Adar, an announcement is made concerning the Shekalim." (Masechet Megillah)

The first of the Four Special Shabbatot is Shabbat Parshat Shekalim. It occurs either on the last Shabbat of the month of Shevat, or on the Shabbat which in that year coincides with Rosh Chodesh Adar, or on a Shabbat early in Adar. A special reading, taken from Parshat KiTisa (Shemot 30:11-16) is appended to the regular Torah reading.

The reading describes a census of the Jewish People which was taken while the Jews were in the Wilderness, after their Exodus from Egypt. The Torah, here and in other places, teaches that it is forbidden to count Jews in the ordinary manner; rather, the People should be called upon to contribute items, which would then be counted.
Continue reading article: http://www.ou.org/chagim/fourshabbatot/shekalim.htm