I’ve been working on a historical short story, and as part of my research I found this site, which has a calculator to tell you the day of the week for any date back to Monday, January 1, 4713 B.C., and forward to Saturday, December 28, 24660198799805 A.D. (but it doesn’t take into account future adjustments for the slowing of the Earth’s rotation — nor, presumably, the swallowing of the Earth by the sun in its red giant phase. And if you’re planning to write a story that takes place after our sun has cooled from a white dwarf to a black dwarf, that shouldn’t happen until sometime after Saturday, December 28, 24660198799805 A.D., so you’re just plain out of luck if you want to be accurate about which day of the week your story happens on. Then again, who’s likely to call you on it if you say it was Sunday, March 15, 73234823625144665 A.D.?)
It automatically adjusts for the switch between the Julian and Gregorian calendars, although you have to manually force Julian when needed for dates in places that were later in switching.
Actually, now that I think about it, I can tell you that March 15 will fall on a Wednesday in 73234823625144665 A.D. It’s rather simple to check, in fact.