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From the above timeline, the dates before September 2010 and between July - September 2011 have the most number of sets in rotation (a total of 9 sets!).
On September 2010 & 2011 though, only 5 sets are legal. A few months from now, the Zendikar block and Magic 2011 Core set will rotate out.
+A few weeks before a set's release, Wizards goes into "spoiler season". The cards for the set are slowly revealed a bunch at a time to the world. This is another reason why the weeks before September are very good for getting back into Magic. Since the cards are revealed in bite-sized groups and the community tends to immediately assess a card's worth the minute it is spoiled, you get to know each card of the set pretty well.
Each card in Magic (except the really old ones from Alpha, Beta, etc.) has a symbol located at the right side just below the artwork. That symbol identifies which set it belongs to.