The paper missal booklets in most churches are changed depending upon the season, or their subscription contract with the publisher. The “good through” dates are on the front cover. There should be a new one to start Advent.
If someone owns their own private missal, then they will not need to get a new one, as they are made for “perpetual use”. There are different versions by different publishers, but all contain the same information. The differences might be things like the order sections are arranged in, or which bible translation is used.
The current Sunday Lectionary schedule is divided into Year A, Year B, and Year C (it is different for weekdays):
The Sunday cycle is divided into three years, labeled A, B, and C. 2008 was Year A. 2009 was Year B, 2010 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 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.
For additional information, see also: usccb.org/about/divine-worship/liturgical-calendar/upload/2013cal.pdf
If you are attending a mass that is Pre-Vatican II (Latin), a different missal is used.