Answer: New Year’s Eve
While launching men and women into space seems inherently high-tech and futuristic, there were many elements of NASA’s Space Shuttle Program that were downright anachronistic by the time the shuttles were retired. One of the side effects of running a spacecraft full of 1980s-era computer chips and 1970s-era code was a very cautionary stance towards events that may have introduced unknown variables or factors the computer system wasn’t programmed to handle correctly.
It was that extreme caution and desire to bring every crew home safely that led NASA to approach the yearly rollover from December 31st to January 1st with a “better safe than sorry” attitude. The engineers were particularly concerned that the Year-End Rollover (YERO) event could cause some disturbance in the flight software that might endanger the crew. Even when they finished conducting extensive tests in 2007 and devised a solution to ensure safety, they still erred on the side of caution.
Over the span of 135 shuttle flights between 1981 and 2011, no shuttle was ever launched, in space, or guided to re-entry on New Year’s in order to avoid any unforeseen problems with the onboard systems.