A student writes:

  Dr. Patt, I have a question about program lab 5.  
  I'm not too sure about the 
  way you explained the equation on how to get 
  the day of the week.  Can you 
  please explain a little more on it.  Thank you.

  <<name withheld>>

Without going through the whole thing, let me just provide the crux:

  Thought #1:  If I say A is a Tuesday, and ask you what day of the week is
  18 days after A, what do you say?  

  Answer: You know that Tuesdays come every seven days.  So, if A is a Tuesday, so
  is A+7, A+14, A+21, etc.  What about A+18?  Well, if I divide by 7, and I get 
  remainder of 0, then I am looking at Tuesday.  Remainder of 1: Wednesday.
  Remainder of 2: Thursday, etc.  18/7 --> remainder of 4.  Therefore: Saturday.

So, I start with January 1, 1901, and count the days in all the years until January
 1 of the year of interest.  Using the thought above, I determine the day of the week
 of that January 1.  Then I repeat the process to get the 1st of the month of interest.  
Finally, I repeat the process to get the day of 
the month of interest.

Got it?

Yale Patt