Historically, this is a tough week for NASA. Tuesday was the anniversary of the Apollo 1 fire (1967), Wednesday was the anniversary of the shuttle Challenger explosion (1986), and this coming Sunday (Feb. 1) is the anniversary of the loss of Columbia during reentry (2003).
So NASA has set today aside as its special day of remembrance of these tragic events. Space.com has a brief writeup on the events here. You might want to consider commemorating them in your own way today as well.