Find day of week from month and year

[John Machin]

Terry Reedy wrote:
>>Laguna wrote:
>>
>>>I want to find the expiration date of stock options (3rd Friday of the
>>>month) for an any give month and year.
> 
> 
>>From year and month (and day=1) get the day of the week (n in [0,6]) of the 
> first of the month using some version of the the standard formula (see 
> below) and look up the third friday date in a precalculated 7-element list, 
> or, with n=0 on Saturday, 3rd Friday is 21-n
> 
> Here is a translation of the guts of a 30-year-old Basic program:
> 
> def friday3(m,y): # ints
>   if m <= 2:
>     m += 12
>     y -= 1
>   d = 1
>   n = d + 2*m + int(.6*(m+1)) + y + y//4 - y//100 + y//400 + 2

Some simplification is possible:

 >>> [2*m + int(.6*(m+1)) for m in range(15)]
[0, 3, 5, 8, 11, 13, 16, 18, 21, 24, 26, 29, 31, 34, 37]
 >>> [(13*m+3)//5  for m in range(15)]
[0, 3, 5, 8, 11, 13, 16, 18, 21, 24, 26, 29, 31, 34, 37]
 >>>


>   n = int((n/7.0- n//7)*7.0 + .5)

n %= 7

>   # n=0 is Saturday, making 3rd Friday the 21st.
>   return 21 - n
> 
> 
>>>Requirements:
>>>d0 = expiration(9, 2005) # d0 would be 16
>>>d1 = expiration(6, 2003) # d1 would be 20
>>>d2 = expiration(2, 2006) # d2 would be 17
> 
> 
>>>>for m,y in ((9,2005), (6,2003), (2,2006)): print friday3(m,y)
> 
> ...
> 16
> 20
> 17
> 
> Terry J. Reedy
> 
> 
>