Roughly what date do NSF Postdoc Fellowship announcements occur?
In another answer, Zach H said that notifications for 2014 went out on January 14th.

In 2015, the date was January 26.

In general, the notifications are sent out a week or so before the coordinated postdoc deadline, the day before which most research universities agree not to require a decision on postdoc offers. http://www.ams.org/profession/employment-services/deadline-coordination/deadline-coordination  This deadline changes from year to year, but it is always toward the end of January/beginning of February.

The idea is that people should have an opportunity to find out whether they've received the NSF before making any other decisions on postdoc offers, and if they were not one of the lucky few getting the NSF, then they should have at least a week to mull over their decision.