In the observance of the annual cycle of Torah Reading, each week is assigned its own portion of Scripture, or what we refer to as a Parashah. The first Parashah of the year for example is B'resheet (Genesis) and this year 5773 it commences on 27 Tishrei or October 13, 2012. There are 54 Parshiyot (Parashah plural), one for each week of a leap year, so that in the course of one year, we complete the entire Torah from B'resheet to D'varim (Genesis to Deuteronomy). A leap-year adds an additional 4 weeks (one month) to the usual 12 months, and we refer to this month as: ADAR II. During non-leap years (regular), there are 50 weeks, as a result some of the shorter Parshiyot are doubled up.
The weeks of Pesach (Passover) and Sukkot (Tabernacles) have different Torah portions assigned to them, so on leap years that leaves 52 weeks to accomplish 54 Parshiyot. This is achieved by doubling up 2 portions, and for non-leap years there remain 48 weeks to accomplish 54 Parshiyot and this is achieved by having 6 double Parshiyot.