In writing, I like to have my days of the week accurate with the year I’m writing in; at the moment I’m putting the final touches on a novel set in 1788, and to get the dates accurate, here’s the website I’ve been using:
Just feed in the year and place you’re wanting, and bob’s your uncle. If you go further back in time, just be aware that calculations of months and days have not always been the same; The Gregorian calendar has gone through some changes since its inception; for instance, there are 10 days missing in October 1582 (the last day of the Julian calendar was Thursday, 4 October 1582 and this was followed by the first day of the Gregorian calendar, Friday, 15 October 1582).
There are also other calendars to consider, e.g. the Roman calendar (http://en.wikipedia.org/wiki/Roman_calendar), Germanic, Icelandic, Asian, Arabic… time has long been calculated. It’s a bit comforting to know, when I have trouble remembering what day of the week it is sometimes!