Why bother learning Doomsday — what makes it worth it?

It turns "what day was that?" into a few seconds of head math, needing zero memorised calendars — and it's a genuinely impressive, learnable skill.

• Party trick: instantly name the weekday of anyone's birthday or a famous date.
• Practical: schedule or reason about dates without a calendar.
• Learnable: it's a fixed algorithm — practice, not talent. With drilling, people reach sub-10-second times.

And it's pure, self-contained modular arithmetic, so it reinforces the mental-math skills next door.