Find day of week from month and year

[Terry Reedy]

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