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.
Private Mod Note
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
Will there be a period in which we can play with cards from both M12 and M13?