In the same vein as yesterday:
Calculation of the Ecclesiastical Calendar — Western, Orthodox, Gregorian, Julian, it’s all here, including information on how the date for Easter was and is calculated.
Medieval Calendar Calculator — Print out nifty one month or full-year calendars for years between 500 and 1582, when the Gregorian calendar came into use in most European countries. You can opt to include feast days, holy days, and saint’s days. I use this one to keep track of my story timeline for IMMORTAL WARRIOR, which takes place roughly between Christmas 1095 and All Hallows, 1096.
Did I mention I can be a bit obsessive, too?