Wizards of the Coast has announced changes in the way blocks will be released for Magic: The Gathering.  Beginning in the fall of 2015, Magic will shift from a three block set followed by a core release to a two block, twice-a-year format.
In the announcement, head Magic designer Mark Rosewater outlined the problems the constant reinvention and evolving nature of the game present the designers as its releases are currently structured.  Rosewater discussed limitations and concerns with the current format including the drawbacks of a three set block format (especially how to keep interest up for the third set); balancing the needs of new players and experienced ones for core sets; the problems of keeping the "metagame" fresh, since players can now communicate and interact more rapidly; the fact that storytelling is slowed by the three set format; and limitations on the ability to shift world settings as frequently as desired. 
Wizards feels this new two block, twice-a-year format will address all of these concerns.  This new format will also lead to the elimination of the core set, and the rotation of Standard format cards every 18 months instead of 24.  The announcement was made now because since Khans of Tarkir will be the last of the three set blocks, there will be an impact on how long cards from that set will be legal for Standard format.
Beginning with the "Blood" set in Fall 2015, the yearly format will include a large fall expansion and small winter expansion set in World #1, then a large spring expansion and a small summer expansion, usually set in World #2.  "The first two sets will be part of one block and the second two sets will usually be part of a second," Rosewater explained, while also saying "Magic is about change and you should expect us to shake this up a bit over time."
The change will also affect the metagame, since the size of Standard will change as well.  Beginning in 2016, Standard format play will be three two-set blocks long rather than two three-set blocks.  Again, this means that the Khans of Tarkir block will only be in Standard for 18 months, as the new schedule takes effect.  Standard will still be made up of five or six expansions, but it will come from three blocks of releases instead of two.