The Lectionary is arranged in two cycles, one for Sundays and one for weekdays.
The Sunday cycle is divided into three years, labeled A, B, and C. 2017 is Year A. 2018 is Year B, 2019 is Year C, etc. In Year A, we read mostly from the Gospel of Matthew. In Year B, we read the Gospel of Mark and chapter 6 of the Gospel of John. In Year C, we read the Gospel of Luke. The Gospel of John is read during Lent and the Easter season in all three years. The first reading, usually from the Old Testament, reflects important themes from the Gospel reading. The second reading is usually from one of the epistles, a letter written to an early church community. These letters are read semi-continuously. Each Sunday, we pick up close to where we left off the Sunday before, though some passages are never read.
The weekday cycle is divided into two years, Year I and Year II. Year I is read in odd-numbered years (2017, 2019, etc.) and Year II is used in even-numbered years (2018, 2020, etc.) The Gospels for both years are the same. During the year, the Gospels are read semi-continuously, beginning with Mark, then moving on to Matthew and Luke. The Gospel of John is read during Christmas, Lent, and the Easter season. For Advent, Christmas, and Lent, readings are chosen that are appropriate to the season. The first reading on weekdays may be taken from the Old or the New Testament. Typically, a single book is read semi-continuously (though some passages are not read) until it is finished and then a new book is started.
The year of the cycle does not change on January 1st, but on the 1st Sunday of Advent (usually late November/early December) which is the beginning of the liturgical year. The liturgical year 2018 will begin on December 3, 2017 and end on December 1, 2018.
In addition to the Sunday and weekday cycles, the Lectionary provides readings for feasts of the saints, for common celebrations such as Marian feasts, for ritual Masses (weddings, baptisms, etc.), for votive Masses, and for various needs. These readings have been selected to reflect the themes of these celebrations.