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.

They're deliberately spaced whole numbers of weeks apart, so they always share one weekday — the doomsday.

For example, 4/4 to 6/6 is exactly 63 days = 9 × 7, so they're always the same weekday; 6/6 to 8/8 is 63 days too. The odd-month and February anchors are placed to line up the same way. The set isn't magic — it's a carefully chosen collection of dates that are all a multiple of 7 apart, which is what lets one "doomsday" cover the whole year.