The first time I flew on a Friday the 13th I had an entire row of 3 seats to myself, a luxury for the moderately long ride from Seattle to Boston. Ever since then I make a point of booking on the 13th if possible, especially overseas flights where a little more elbow room is most welcome. I've made two trans-Pacific flights so far on the 13th, and the strategy paid off both times. An empty adjacent seat is a real rarity these days.
None of those were on flight 666, though. I wonder if ticket agents get tired of "is this non-stop, straight to HEL?"