A week of ill omen?

Something lurking in the shadow cast by a waxing moon on the division of the month, the Ides, beware, the soothsayer said.

The ancient Roman calendar marked each month with kalends, nones, and ides.
In March, May, July and October, the ides fall on the 15th day.

In every other month they fall on the 13th.

As the name implies, the ides divides.