There is something unusual about today's date, October 24, 2026. Beyond being another Saturday on the calendar, it has a mathematical connection that students may recognise from their school textbooks.
October 24 is being observed as Pythagorean Theorem Day, a rare occasion when the numbers representing the month, day and year satisfy one of geometry's most familiar principles.
The date, written as 10/24/26, produces the numbers 10, 24 and 26. These three numbers can represent the sides of a right-angled triangle.
What makes the date particularly interesting is that the next such occurrence, under the same date convention, will be on March 4, 2105 — nearly 79 years from now.
The Pythagorean theorem states that in a right-angled triangle, the square of the longest side, called the hypotenuse, equals the sum of the squares of the other two sides.
For today's date, the calculation works as follows:
Month: October = 10
Day: 24
Last two digits of the year: 26
Square of 10 = 100
Square of 24 = 576
Sum = 676
Square of 26 = 676
Since 100 + 576 = 676, the numbers satisfy the Pythagorean theorem.
A set of three positive whole numbers satisfying this relationship is known as a Pythagorean triple.
The next date following this pattern will be March 4, 2105, written as 3/4/05.
The numbers 3, 4 and 5 form another Pythagorean triple, since 3² + 4² = 5².
The last such date before October 24, 2026, occurred on July 24, 2025, represented by the numbers 7, 24 and 25.
Other examples from recent years include August 15, 2017 (8, 15, 17) and December 16, 2020 (12, 16, 20).
The Pythagorean theorem is taught in school mathematics and is used in geometry, construction, surveying, engineering and several other fields.
While students usually encounter the theorem through textbook exercises, October 24 offers an example of how mathematical relationships can appear in everyday situations, including calendar dates.
The University of Oxford's Mathematical Institute has also highlighted October 24, 2026, as Pythagorean Theorem Day, encouraging students to explore right-angled triangles and number patterns.
For students and mathematics enthusiasts, the date offers a simple reminder that mathematics is not confined to the classroom. Sometimes, an interesting mathematical relationship can be found simply by looking at the calendar.