Determining if a particular date is a holiday is a global problem that has several solutions among iSeries shops. Some shops set up a calendar file of some sort, keyed by date, with a field flagging it as a holiday. This is OK, as long as you remember to generate it every year. I prefer a program that receives a date, of any year, and determines if it is a holiday. With an algorithmic program, you would not need to remember to generate new records in a calendar file every year, and you wouldn't use up disk space.

There are several things to consider when determining if a date is a holiday:

• Some holidays are dated--that is, they occur on a specific date, such as 12/25, 11/11, 1/1 or 7/4. Sounds easy enough, but if they fall on the weekend, your company may not observe them on their actual date. Then, you have to determine when your company observes the holiday during the work week.
• Other holidays float--that is, they occur on the nth xxxday of a month, such as the third Monday of January, the fourth Thursday of November, or, even trickier, the last Monday of May.
• Then there's Good Friday and Easter, but even that can be determined algorithmically.

I have marveled at the lengths to which some code goes to determine if a day is the nth xxxday of the month, sometimes involving complicated comparison and counting calculations. I have seen similar complex coding for determining observed holidays. I have discovered that such complicated processing is not necessary. To determine if a date is a holiday, you need only two pieces of information:
 

1. The date itself (specifically the month and day)
2. The day of the week

Determining the day of the week in RPG is simple:

This produces 1 for Monday through 7 for Sunday.

For dated holidays like 7/4, 11/11 and 12/25, the actual date is the holiday. If that holiday falls during the work week, you're set. But if it falls on the weekend, you have everything you need to determine when the holiday might be observed by your company during the work week. Normally, if a holiday falls on Saturday, it is observed on Friday; likewise, a Sunday or Tuesday holiday is observed on Monday. You can then deduce, for example, that Friday, 7/3, or Monday, 7/5, and so on, could be observed holidays.

Floating holidays are just as easy to determine. There is one simple fact to keep in mind when determining if a date is the nth xxxday, and this fact is easy to overlook: There are only seven xxxdays (Monday through Sunday). It follows then that the first xxxday of the month will be on one of the first seven days. We can now determine easy ranges of seven days that we need.

If you have a September Monday between 9/1 and 9/7, then you have Labor Day. If you have a November Thursday between 11/22 and 11/28, it is Thanksgiving. Likewise, a May Monday between 5/25 and 5/31 is Memorial Day.

Other useful ranges are 11/23 through 11/29 to determine the Friday after Thanksgiving (that Friday will follow the fourth Thursday), and 11/2 through 11/8 to determine Election Day (the first Tuesday in November on or after 11/2).

As for Easter and Good Friday, there is an accepted algorithm called the Mallen Algorithm. It is easy to implement in RPG, COBOL, or Java. Remember also that Easter will always fall somewhere between 3/22 and 4/25.

Now, what about calculating a holiday date? In other words, how would you code a function to answer "When is Thanksgiving in 2002?"

For dated holidays, that's easy. You already know the answer. "When is Christmas in 1999?" 12/25/1999, of course. If this falls on the weekend or even the middle of the week, when might your company observe it? This involves some work, but not much. Simply determine the day of week of the dated holiday. If it is 6 (Saturday), subtract a day from the date. For 7 (Sunday), add a day. In 1999, Christmas is on Saturday, so it is observed Friday, 12/24/1999.

For the floating holidays, it's not that difficult, either. We need the following information:

• The month and year. We know these already.
• x: The xxxday we are looking for. For Thanksgiving, x would be 4 (Thursday)
• d: The first day of the range. For Thanksgiving, d would be 22, because the fourth Thursday falls somewhere between the 22^nd and the 28^th.
• n: The day of the week that d falls on, for the given month and year.

Now, to determine h, the day the holiday falls on, we use the following formula:

So for Thanksgiving in 2003, we have:

• Month and Year: 11/2003
• x = 4
• d = 22
• n = Weekday of 11/22/2003 = 6 (Saturday)

h = 22 + ((4 + 7) - 6)
h = 22 + (11 - 6)
h = 22 + 5
h = 27

Thanksgiving in 2003 is 11/27.

The sample program below receives a date (numeric CYMD) and determines if it is a holiday. If it is, it returns a code indicating which holiday as well as a text description of the holiday.

Doug Eckersley is the iSeries programmer with a homebuilder in Columbus, Ohio. He has been programming on the AS/400 for ten years. He is certified by IBM.  He can be reached by email at This email address is being protected from spambots. You need JavaScript enabled to view it..