The poem starts with the fear of Friday the 13th and quickly gently mocks it. The speaker points out that the superstition is already called ancient when it first appears in writing, and that real ancient people had far subtler ways of sensing a bad day: three crows landing to meet their shadows, or a wild columbine pushing its red flowers up where it doesn't belong. These signs come from close attention to the world, not from a number on a calendar.
Then the tone turns wry and a little rueful. "You & I" are the descendants of people who preferred sudden, dramatic revelation to patient weighing of evidence, and the missing thirteenth floors and boardrooms full of Judases show how silly and how human that is. The detail about millions staying in bed suggests that avoiding the world feels safer than facing it. The ending is quiet and strange: an empty sky, a moon described as a fish-belly, and a question that feels uneasy yet curious. The mood is thoughtful, dry, and slightly eerie.