In the match season 2012-2013 ERAS opened for applicants and residency programs on September 15th 2012. That was a change from the usual date of September 1st every year. We reported that
in March 2012.
For the 2013-2014 season, the official timeline is not out yet (even though we are at the end of March 2013) however ERAS Twitter Account
tweeted in a response to somebody asking that again the opening date this year will be September 15, 2013.
The delayed opening of ERAS gives some extra time for applicant to complete their applications such as those who are waiting for the Step 2 CS results or those who are awaiting to finish their clinical clerkships.
Although, you can still apply later in October or November but over the years it has been shown that if you apply late, you'll get less interview invitations, specially in the case of IMGs. More on that if you check the Match-Timeline