Ravens in a snowstorm near Kuhmo, Finland

© Frans Lemmens/Alam

Once upon a midafternoon dreary…

To the superstitiously inclined, this flock of ravens—pictured battling a snowstorm in eastern Finland—nothing short of a bad omen, while to others it's just a bunch of beaks. Likewise, today is just another flip of the calendar to some, while Friday the 13th consigns others to a day of dread—or at least of relative inactivity to avoid potential mishaps.

The notion of Friday the 13th as doubly jinxed is only a century or two old, but Friday and the number 13 have each long been pegged as unlucky. Old Norse and Christian traditions both tell of suppers spoiled by a malign 13th guest (Loki the trickster and Judas the betrayer, respectively), while nautical lore says a voyage that dares begin on a Friday is doomed to sink to the watery depths. The popular mind began combining the number with the weekday to posit the mother of all bad days in the 1800s, but some historians say it was with the popular 1907 novel 'Friday the 13th'—where a conniving stockbroker incites a Wall Street crash on the titular date—that the compounded phobias became an enduring meme.