A lectionary is a list of readings. As a spiritual discipline, a person may simply open The Bible at the beginning, and start reading. They might read a chapter or more each day. The weakness of this kind of reading is that it is the reading style of our time, the method for reading a novel, or even a text book. It assumes a narrative thread from beginning to end. However, a text book is often not read from cover to cover. It may be designed as a resource with discrete sections to be consulted at appropriate times.
The Bible is even less novel-like. With 39 "books" in the Hebrew Scriptures, and 27 in the Christian Scriptures, there are multiple authors, times, geographic locations, and theological perspectives represented. This considers only the main collection (Canon) of the books common to most Christian traditions. There are also the books not present in the Hebrew Scriptures or "Old Testament" which are often known as the deutero-canonical books. How does one read all this and make sense of it?
Christian groups have traditionally created lists of texts that are considered important to read. They sketch out some of the key planks of that group's tradition, and its understanding of the Christian faith.
One well known modern lectionary is the Revised Common Lectionary, which is used by many churches world wide. It divides the bible over a three year period, based around the Gospels of Matthew, Mark and Luke. The readings are chosen to reflect the cycle of church year as it progresses from the hope for a Messiah (Advent), through Christmas, and on to Easter. Each week also has a reading from the Hebrew Scriptures, the Psalms, and from the letters of the New Testament. The Gospel of John is used in each year around the times of the major festivals. There are often readings assigned for special days which do not occur on a Sunday.
Many ministers preach from a lectionary. It provides a discipline which works against the temptation to avoid uncomfortable subjects and concentrate on favourite themes.
A lectionary provides an overview of the Christian tradition. Unfortunately, it also represents a particular theological and historical outlook. Some people point out, for example, that women's stories, often already marginalised in Scripture are further submerged by the RCL . The lectionary is also constructed of short readings, excerpts from the whole, so that some parts of the bible will never be read in public worship under this scheme. It also means that the wider flow of a narrative is interrupted, and perhaps divided in ways never anticipated by the authors. In their own devotions, many people will at least read from the end of the previous week's readings to the end of the designated readings of the current week, in some attempt to overcome this disintegration of the narrative.