© 2008 Michael Thompson Algorithm for Calculating Floating Holidays [ Main Index ]

Sample Implementation:

Holiday	Definition
ML King	3rd Monday of Jan	Mon, Jan 21st
Presidents Day	3rd Monday of Feb	Mon, Feb 18th
Memorial Day	Last Monday of May	Mon, May 26th1
Labor Day	1st Monday of Sep	Mon, Sep 1st
Columbus Day	2nd Monday of Oct	Mon, Oct 13th
Election Day	Tuesday after 1st Monday of Nov	Tue, Nov 4th2
Thanksgiving Day	4th Thursday of Nov	Thu, Nov 27th

Purpose:

To determine the date of a floating holiday for a given year, i.e. the date of a holiday that occurs on the first Monday of the month (like Presidents Day) or a holiday that occurs on the fourth Thursday of the month (Thanksgiving)

Method:

1. Set the following values corresponding to the holiday's target day of the week (0=Sunday, 1=Monday, etc.), the month (Jan=1) and the desired year:

   
   nTargetday=1
   nMonth=2
   nYear=2003
   

2. Determine from the definition of the holiday the earliest possible date in the month on which it could occur:

   
   nEarliestDate = 1 + 7 * (nTh - 1)
   

   where nTh is the number of the occurence of the target day, i.e. given "the third Friday", nTh=3; given "the first Monday" nTh=1

3. Determine the weekday of the earliest possible date in the month on which the holiday could occur in the given year. Since this is language-specific, here are a few examples:
   in PHP:

   
   $nWeekday = date("w",mktime(0,0,0,$nMonth,$nEarliestDate,$nYear));
   

   in Java/Javascript:

   
   d = new date(nYear,nMonth,nEarliestDate);
   nWeekday = d.getDay();
   

   in C:

   
   struct tm mtm;
   memcpy(&mtm,localtime(time()),sizeof(struct tm));
   mtm.tm_mon = 1 + nMonth;
   mtm.tm_mday = nEarliestDate;
   mtm.tm_year = nYear - 1900;
   mktime(&mtm);
   nWeekday = mtm.tm_wday;
   

   (Note: If you need a version that works for 2038 through 2105 use a 64-bit version of mktime)

4. Find the offset between the target weekday and weekday of the earliest possible date in the given year:

   
   if( nTargetday==nWeekday ) nOffset = 0;
   else
   {
     if( nTargetday<nWeekday ) nOffset = nTargetday + (7 - nWeekday);
     else nOffset = (nTargetday + (7 - nWeekday)) - 7;
   }
   

5. Calculate the actual date of the holiday using the earliest possible date and the offset for the given year:

   in PHP:

   
   $tHolidayDate = mktime(0,0,0,$nMonth,$nEarliestDate + $nOffset,$nYear);
   

   in Java/Javascript:

   
   tHolidayDate = new date(nYear,nMonth,nEarliestDate + nOffset);
   

   in C:

   
   mtm.tm_mon = 1 + nMonth;
   mtm.tm_mday = nEarliestDate + nOffset;
   mtm.tm_year = nYear - 1900;
   tHolidayDate = mktime(&mtm);
   

Memorial Day

To follow the rule "last Monday in May" we calculate the fourth Monday as shown above and then add one week if there are five Mondays in May.

Election Day

To follow the rule "the Tuesday after the first Monday in November" we calculate the first Monday as shown above and then add one day.