I find this helps in the understanding the Responsorial Psalms & the Liturgical Cycles for the Sunday Missal and the Week-DAY Missal.
• The Lectionary is arranged into two cycles, one for Sundays and one for weekdays.
• The Sunday cycle is divided into three years, labeled A, B, and C. 2005 is Year A, 2006 will be Year B, 2007 will be Year C, and so on.
• The Liturgical Year begins on the 1st Sunday of Advent (usually late November) and ends with the Feast of Christ the King.
• 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 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 (2003, 2005, etc.) and Year II is used in even-numbered years (2002, 2004, 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 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 (i.e. some passages are not read) until it is finished and then a new book is started.