Friday the Thirteenth
The Gregorian calendar 
follows a pattern of leap years which repeats every 400 years. Thereare 4,800 months in 400 years, so the 13th of the month occurs 4,800 times in this interval. The number of times the 13thoccurs on each weekday is given in the table below. As shown by Brown (1933), the thirteenth of the month is slightly morelikely to be on a Friday than on any other day.
| Day | Number of 13s | Fraction |
| Sunday | 687 | 14.31% |
| Monday | 685 | 14.27% |
| Tuesday | 685 | 14.27% |
| Wednesday | 687 | 14.31% |
| Thursday | 684 | 14.25% |
| Friday | 688 | 14.33% |
| Saturday | 684 | 14.25% |
References
Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed. New York: Dover, p. 27, 1987. Brown, B. H. ``Solution to Problem E36.'' Amer. Math. Monthly 40, 607, 1933. Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed. Cambridge, England: Cambridge University Press, pp. 14-15, 1992.
- Immersion
- Impartial Game
- Implicit Function
- Implicit Function Theorem
- Implies
- Impossible Figure
- Impredicative
- Improper Integral
- Improper Node
- Improper Rotation
- Impulse Pair
- Impulse Symbol
- Inaccessible Cardinal
- Inaccessible Cardinals Axiom
- Inadmissible
- In-and-Out Curve
- Incenter
- Incenter-Excenter Circle
- Incidence Axioms
- Incidence Matrix
- Incident
- Incircle
- Inclusion-Exclusion Principle
- Inclusion Map
- Incommensurate