Such missions must be accomplished during moonless nights (i.e. Moon below horizon) so window of opportunity is every 25 or so days (2-3 moonless nights every month) Moonless nights were around May 1st. Allegedly, first mission day earlier was aborted.
I believe it was something leaked by Wikileaks (gitmo files)April 25 that triggered action, so the mission could not wait the next moonless night at the end of May.