What is the Y Combinator cycle?
My answer to What is the Y Combinator cycle? on Quora
The winter program generally runs from January until March, with application deadlines the previous October (this year the deadline was was October 21st). Interview invites were sent out on November 4th, and interviews themselves in Mountain View were November 15th-19th. Teams get a yes or no answer the day of their interview.
The summer program runs from June until August, with application deadlines in March. For summer 2013 the deadline was March 29th, invites went out on April 16th, and interviews were April 26th-30th.