Friday the 13th

by

Jim Wilson

Let's start by number the days of the year 1 to 365 for non-leap years; 1 to 366 for leap years. Then it is quite easy to accumulate a list to enumerate which day of the year corresponds to the 13th of each month. For example January 13 is the 13th day of the year, February 13 is the 44th day of the year, March 13 is the 72nd day (in non-leap years) or the 73rd day (leap years).The week is seven days long and so counting off by 7s and keeping the remainders could give us a way of telling which day of the week each of these occurred IF we knew what day of the week January 1 was on.

Non-Leap Year ..................................Leap Year

 January 13 6 13 6 February 44 2 44 2 March 72 2 73 3 April 103 5 104 6 May 133 0 134 1 June 164 3 165 4 July 194 5 195 6 August 225 1 226 2 September 256 4 257 5 October 286 6 287 0 November 317 2 318 3 December 347 4 348 5

Now, all that remains to to examine the cases for January 1 .

NON LEAP YEAR

 If January 1 is on Then Fridays will be a Friday 13th in the months of Sunday 6 January October Monday 5 April July Tuesday 4 September December Wednesday 3 June Thursday 2 February March November Friday 1 August Saturday 0 May

LEAP YEAR

 If January 1 is on Then Fridays will be a Friday 13th in the months of Sunday 6 January April July Monday 5 September December Tuesday 4 June Wednesday 3 March November Thursday 2 February August Friday 1 May Saturday 0 October

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