Weekday
The day of the week 
(numbered according to 
for Sunday, 
for Monday, etc.) for a given day of the month 
,month 
, and year 
can be determined from the simple equation
where
is the Floor Function, months are numbered beginning with March and dates in January andFebruary considered to be the 11th and 12 months of the previous year (Uspensky and Heaslet 1939, Vardi 1991).A more complicated form is given by
where
for Sunday, 
for Monday, etc. and the numbers assigned to months, centuries, and yearsare given in the tables below (Kraitchik 1942, pp. 110-111).| Month | |
| January | 1 |
| February | 4 |
| March | 3 |
| April | 6 |
| May | 1 |
| June | 4 |
| July | 6 |
| August | 2 |
| September | 5 |
| October | 0 |
| November | 3 |
| December | 5 |
| Gregorian | |
| Century |  |
| 15, 19, 23 | 1 |
| 16, 20, 24 | 0 |
| 17, 21, 25 | 5 |
| 18, 22, 26 | 3 |
| Julian | |
| Century | |
| 00, 07, 14 | 5 |
| 01, 08, 15 | 4 |
| 02, 09, 16 | 3 |
| 03, 10, 17 | 2 |
| 04, 11, 18 | 1 |
| 05, 12, 19 | 0 |
| 06, 13, 20 | 6 |
| Year | Y | ||||||||
| 00 | 06 | 17 | 23 | 28 | 34 | 45 | 0 | ||
| 01 | 07 | 12 | 18 | 29 | 35 | 40 | 46 | 1 | |
| 02 | 13 | 19 | 24 | 30 | 41 | 47 | 2 | ||
| 03 | 08 | 14 | 25 | 31 | 36 | 42 | 3 | ||
| 09 | 15 | 20 | 26 | 37 | 43 | 48 | 4 | ||
| 04 | 10 | 21 | 27 | 32 | 38 | 49 | 5 | ||
| 05 | 11 | 16 | 22 | 33 | 39 | 44 | 50 | 6 | |
| 51 | 56 | 62 | 73 | 79 | 84 | 90 | 0 | ||
| 57 | 63 | 68 | 74 | 85 | 91 | 96 | 1 | ||
| 52 | 58 | 69 | 75 | 80 | 86 | 97 | 2 | ||
| 53 | 59 | 64 | 70 | 81 | 87 | 92 | 98 | 3 | |
| 54 | 65 | 71 | 76 | 82 | 93 | 99 | 4 | ||
| 55 | 60 | 66 | 77 | 83 | 88 | 94 | 5 | ||
| 61 | 67 | 72 | 78 | 89 | 95 | 6 |
References
Kraitchik, M. ``The Calendar.'' Ch. 5 in Mathematical Recreations. New York: W. W. Norton, pp. 109-116, 1942. Uspensky, J. V. and Heaslet, M. A. Elementary Number Theory. New York: McGraw-Hill, pp. 206-211, 1939. Vardi, I. Computational Recreations in Mathematica. Reading, MA: Addison-Wesley, pp. 237-238, 1991.
- Lotka-Volterra Equations
- Low-Dimensional Topology
- Lower Bound
- Lower Denjoy Sum
- Lower Integral
- Lower Limit
- Lower Sum
- Lower-Trimmed Subsequence
- Lowest Terms Fraction
- Loxodrome
- Lozenge
- Lozenge Method
- Lozi Map
- Lp'-Balance Theorem
- L-Polyomino
- L-Series
- L-System
- Lucas Correspondence
- Lucas Correspondence Theorem
- Lucas-Lehmer Residue
- Lucas-Lehmer Test
- Lucas' Married Couples Problem
- Lucas Number
- Lucas Polynomial
- Lucas Polynomial Sequence