There is currently one rotation per year for Standard. It is the first friday of October. At that time, the new large fall set becomes legal, the oldest block rotates out, and the previous year's core set rotates out. Since the new core set of each year comes out and becomes legal during the summer, there is a period of a few months where two core sets are legal at the same time.
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DCI Lvl 1 Judge
Madness, My Sadness I Am My Own Savior -Iced Earth
The following card sets are permitted in Standard tournaments:
•Scars of Mirrodin (until October 5, 2012)
•Mirrodin Besieged (until October 5, 2012)
•New Phyrexia (until October 5, 2012)
•Magic 2012 (until October 5, 2012)
•Avacyn Restored (effective May 4, 2012)
•Magic 2013 (effective July 13, 2012)
Yes, there will be a time when both M12 and M13 are in Standard together.