The Java class GregorianCalendar has some oddities. It is also unable to calculate historical dates for countries like England that did not implement the Gregorian calendar on October 15, 1582. This note discusses the Julian and Gregorian calendars, shows how to use the class to calculate the date of week for modern times (dates since 1927) and historical times for countries that implemented the Gregorian Calendar on October 15, 1582.

The notes are best read on a desktop epub reader. The note is not copy-protected so you can cut and paste to your java build environment.