March 13, 2017 was the final receipt date for new H-2B worker petitions requesting an employment start date before October 1, 2017.

What Happens After Reaching the Cap
Except as noted below, USCIS will reject new H-2B petitions received after March 13 that request an employment start date before October 1, 2017.
Petitions That Are Exempt from the Cap
USCIS will continue to accept H-2B petitions that are exempt from the congressionally mandated cap. This includes the following types of petitions:

• Current H-2B workers in the U.S. petitioning to extend their stay and, if applicable, change the terms of their employment or change their employers;
• Fish roe processors, fish roe technicians and/or supervisors of fish roe processing; and
• Workers performing labor or services from November 28, 2009 until December 31, 2019, in the Commonwealth of Northern Mariana Islands and/or Guam.

Source: USCIS Reaches the H-2B Cap for Fiscal Year 2017 | USCIS