Here's some more speculation:
In 2007, awardees were notified on Monday, April 2, 2007 and requested to respond by Monday, April 9, 2007.
In 2009, awardees were notified on Tuesday, March 31, 2009 and requested to respond by Tuesday, April 7th, 2009.
Based solely on two data points alone, it seems that people are given one full week to either accept or decline the NDSEG fellowship.
This year, according to NeuroJoeBruin's c&p, awardees were requested to respond by Tuesday, April 26, 2011.
I wonder if the award emails were actually suppose to go out on Tuesday, April 19, 2011, but someone accidentally sent them on Saturday...