The readings of the Liturgy of the Word during Sunday liturgy in the Catholic Church (as also is the case in some mainline churches) is arranged in a three year cycle:
Cycle A: From the Gospel of Matthew
Cycle B: From the Gospel of Mark
Cycle C: From the Gospel of Luke
How to find the respective cycle for a given year?
Divide the year (eg.2011) by 3. If the reminder is 1 then we follow the readings of Cycle A. If the reminder is 2 then we use the readings of Cycle B. If the reminder is 0 (that is, it is divisible by 3) then Cycle C.
To get the reminder, there is a short-cut: add all the digits of the numerical year (2+0+1+1=4). Divide the value by 3 (4/3).
In 2011 we are following the Cycle A. Next year (2012) will be Cycle B. And 2013 will be cycle C.