If people do fly less on Friday 13th it is a good thing. Means airlines have to drop the fares

As for other bad luck days, I guess 9/11 is one of them. I've flown on 9/11 itself or the days surrounding it in '02, '03 and '04 and have found the airports pretty stressful on all occassions. To be honest I doubt many people, especially in the UK realise they when booking September 11th flying the 'significance' of the date now...