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What Day Is the Thirteenth? (illustration)
FILE No.96Unsolved

What Day Is the Thirteenth?

Difficulty ★★☆about 4 minLogic PuzzleWeekdays7-day cycle
The 1st was a Sunday. The crime happened on the 13th. What day is it?

Case Record

A crime happened on the 13th of a certain month. The culprit says, "I don't remember what day it was." But his notebook says the 1st of that month was a Sunday.

The days of the week repeat every 7 days (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday).

Your question

What day of the week was the 13th?

Case Files

Evidence

Diagram

Every seven daysDateDay1stSunday8thSunday15thSunday13thFriday (2 days before the 15th)
Diagram

Timeline

  1. Sunday.
  2. The crime happens.

Hints

If you get stuck, open the hints one at a time, in order.

Open hint 1

If the 1st is a Sunday, which other days are Sundays?

Open hint 2

Which Sunday is closest to the 13th?

Open hint 3

How far back is the 13th from there?

Solve It

Your question

What day of the week was the 13th?

Choose the answer closest to the truth.

Share: I'm solving this

The Truth

Friday

The answer is: Friday.

If the 1st is a Sunday, so are the 8th and the 15th. The 13th is two days before the 15th, so it is Friday.

Another way: the 13th is 12 days after the 1st. 12 = 7 + 5, so count 5 days forward from Sunday: Monday, Tuesday, Wednesday, Thursday, Friday.

The 7-day cycleThe 1st, 8th and 15th are SundaysThe 13th is 2 days before the 15thTwo days before Sunday = Friday
Diagram of the solution

Key Points

  • The same weekday repeats every 7 days
  • Count from the nearest known date
  • The 13th is 12 days after the 1st

Why people get fooled

Counting day by day, it is easy to slip up.

Similar tricks

  • The 7-day-cycle type
  • The nearest-Sunday type

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