The idea behind the 3 year cycle is to cause as much of the Gospels as possible to be read in each rotation–at least when it comes to the “important” parts, meaning the events, principle teachings, parables, miracles, etc. that most express our faith. That’s the 3 year Sunday cycle. The 2 year weekday cycle is different, but somewhat related.
First, here’s the short answer: The idea is that the “principle” Gospel readings (the events that are most familiar to us) will all be read as much as possible in 3 years. It won’t always work out that they are in order especially because the seasons themselves don’t correspond to the chronology of Christ’s life. In other words, in the calendar year, the feast of the Transfiguration occurs after the feast of Easter–but of course, that’s not how they happened. That’s why we read Gospels that help us to prepare for Easter during Lent, even though before Ash Wednesday, we might have read selections that occurred earlier chronologically.
Here’s the long answer:
The idea behind the 3 year cycle is to cause as much of the Gospels as possible to be read in 3 years; and to have readings appropriate to the liturgical season or feast
Here’s a selection from the Introduction to the Lectionary:
Part II Lectionary for Sundays and Feasts
Readings for Sundays and feasts have been arranged according to two principles which are called “semi-continuous” or “thematic.” The different seasons of the year and the themes of each liturgical season determine which principle applies in specific cases.
The Old and New Testament readings best harmonize when their relationship is self-evident, that is, when the events and teachings of the New Testament are more or less explicitly related to those of the Old. The Old Testament readings in this lectionary have been chosen primarily because of their relationship to the New Testament selections, especially the gospel reading.
Common themes provide another kind of harmonization among the readings for each Mass. Seasons which best illustrate this principle are Advent, Lent, and Easter, each of which has its own spirit and message.
The Sundays of the year [ie Ordinary Time]…have no particular theme. The epistle and gospel readings for these days are arranged semi-continuously, while the Old Testament readings have been chosen because of their relationship to the gospel passages.
In other words:
During the liturgical seasons and feast days, the Gospels are chosen because they relate to that theme (Resurrection during Pascaltide, awaiting during Advent, etc.).
During the Sundays of Year (the Ordinal/Ordinary ie “numbered” Sundays) the idea is to read as much of the principle Gospels selections as possible (which have not been read during the seasons). The emphasis here is on the “important parts” like the parables or the healing miracles, so that those who attend only Sunday Mass will hear all those “important parts” in the course of 3 years.
Most of the above applies to the Sunday readings. The weekday readings on the other hand are much more “continuous,” especially during Ordinal Time.
