Friday the 13th Calculator

Find all Friday the 13th dates in any year or range of years. Enter values for instant results with step-by-step formulas.

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Formula

Check if the 13th day of each month falls on a Friday (day of week = 5)

The calculator iterates through every month in the specified range and checks whether the 13th day of that month falls on a Friday using standard day-of-week calculation. Every year has at least 1 and at most 3 Friday the 13ths, with an average of approximately 1.72 per year.

Worked Examples

Example 1: Friday the 13ths in 2024

Problem: Find all Friday the 13th dates in the year 2024.

Result: 2024 has 2 Friday the 13ths: September 13 and December 13

Example 2: Three Friday the 13ths in One Year

Problem: Find a year between 2020-2030 that has three Friday the 13ths.

Solution: Check each year:\n2023: Jan 13(Fri), May 13(Sat), Oct 13(Fri) = 2\n2024: Sep 13(Fri), Dec 13(Fri) = 2\n2025: Jun 13(Fri) = 1\n2026: Feb 13(Fri), Mar 13(Fri), Nov 13(Fri) = 3\n2026 starts on Thursday (non-leap year), producing 3 occurrences

Result: 2026 has 3 Friday the 13ths: February, March, and November

Frequently Asked Questions

How many Friday the 13ths occur in a typical year?

Every year has at least one Friday the 13th and can have at most three. The average across the Gregorian calendar cycle is approximately 1.72 Friday the 13ths per year. Years with three Friday the 13ths are relatively uncommon and require specific calendar configurations. For a non-leap year, three Friday the 13ths occur when January 1 falls on a Thursday, placing Friday the 13th in February, March, and November. For a leap year, three Friday the 13ths occur when January 1 falls on a Sunday, placing them in January, April, and July. The pattern of Friday the 13ths repeats exactly every 400 years in the Gregorian calendar, with exactly 688 occurrences in each 400-year cycle, confirming the 1.72 annual average.

Why is Friday the 13th considered unlucky?

The superstition around Friday the 13th combines two separate unlucky traditions: the fear of the number 13 (triskaidekaphobia) and the belief that Fridays are unlucky days. The number 13 has been considered unlucky since ancient times, possibly because it follows the complete number 12 (12 months, 12 zodiac signs, 12 apostles). Fridays were considered unlucky in Christian tradition because Jesus was crucified on a Friday. One popular theory connects the superstition to October 13, 1307, when King Philip IV of France ordered the mass arrest of the Knights Templar, though historians debate whether this event actually influenced the modern superstition. The specific combination of Friday and 13th as doubly unlucky appears to have gained widespread cultural traction in the 19th and early 20th centuries, reinforced by Thomas Lawson 1907 novel about a stock market crash on that date.

What is the mathematical pattern behind Friday the 13th occurrences?

Friday the 13th follows a deterministic pattern based on the Gregorian calendar 400-year cycle, which contains exactly 146,097 days (an exact multiple of 7, ensuring the pattern repeats). Within this cycle, the 13th of each month falls on a Friday more often than any other day of the week. Specifically, the 13th falls on Friday 688 times, on Saturday and Monday 684 times each, and on other days varying amounts. The months most likely to start on a Sunday produce a Friday the 13th, and the distribution is not perfectly uniform due to the leap year rules. Two consecutive months can both have Friday the 13ths when a non-leap year has February and March starting on the same day of the week, which happens because February has exactly 28 days (4 complete weeks) in non-leap years.

What notable events have occurred on Friday the 13th throughout history?

Several significant historical events have coincided with Friday the 13th, though statisticians note this is expected given how frequently the date occurs. On Friday, October 13, 1307, King Philip IV of France ordered the arrest of hundreds of Knights Templar. On Friday, November 13, 1970, a massive cyclone struck Bangladesh (then East Pakistan), killing an estimated 300,000 to 500,000 people. On Friday, October 13, 1972, a Uruguayan Air Force plane carrying a rugby team crashed in the Andes, leading to the famous survival story later documented in the book and film Alive. On Friday, January 13, 2012, the cruise ship Costa Concordia ran aground off the Italian coast. On Friday, November 13, 2015, coordinated terrorist attacks struck Paris. However, research consistently shows that accident and disaster rates on Friday the 13th are statistically no different from any other day.

Can February and March both have Friday the 13th in the same year?

Yes, February and March can both have Friday the 13th in the same year, but only in non-leap years. This occurs because February in a non-leap year has exactly 28 days (four complete weeks), which means March 1 falls on the same day of the week as February 1. Consequently, if the 13th of February is a Friday, the 13th of March will also be a Friday. This happens whenever a non-leap year begins on a Thursday, as January 1 being Thursday places February 13 and March 13 on Fridays. Recent years when this occurred include 2009, 2015, and 2026. This phenomenon cannot happen in a leap year because February has 29 days in leap years, breaking the day-of-week alignment between February and March. When February and March both have Friday the 13ths, they are the closest possible consecutive occurrences, only 28 days apart.

What is the longest possible gap between two Friday the 13ths?

The longest possible gap between two consecutive Friday the 13ths is 14 months (approximately 427 days). This maximum gap occurs when a year has only one Friday the 13th falling in a month between June and October, and the following year does not have its first Friday the 13th until the corresponding month has passed. For example, if one year has its only Friday the 13th in August, and the next year first Friday the 13th is in October, the gap would be 14 months. The shortest possible gap between two Friday the 13ths is 28 days, occurring only when February and March both have Friday the 13th in a non-leap year. Other common gaps include approximately 91 days (3 months), 119 days (about 4 months), and 210 days (about 7 months), depending on the specific calendar year configuration.