The deadline is April 15 for submitting requests for XSEDE allocations that would begin on June 1.
There have been several policy changes for this round of submissions, as well as new resources available, all of which are summarized on the XSEDE website.
XSEDE (the Extreme Science and Engineering Discovery Environment) is an NSF-funded project offering compute resources to researchers across the country.
The XSEDE website also contains an overview of the allocation process.