actually it was Jan 20 1981 when they were relesed
That was the date Reagan took office and the Iranians didn't want to have him pissed at them
"Various allegations concerning a deal between the Reagan kitchen cabinet and Iran in order to delay the release of the hostages have been made over the years, although Senate and House investigations in the 1990s concerning this October surprise conspiracy could find no evidence of these claims. In any case, the hostages were released twenty minutes after Reagan's inaugural address."
http://en.wikipedia.org/wiki/Iran_hostage_crisis