In the US and Canada, a number of public holidays and observances occur on particular weekdays of a month, rather than on specific calendar dates (day of month). This is apparently done in many instances to schedule their occurrence close to a weekend. Unfortunately, the laws defining these holidays usually specify the weekday of a particular calendar month. As a result, the actual occurrence of these holidays is haphazard from year to year, and the intervals between such holidays vary from year to year, making the planning of work, vacation, and academic schedules difficult. —Walter Ziobro