Ash Wednesday, of course, occurs 46 days before Easter. Now, calculating Easter is based on astronomy and tables. Easter falls on the Sunday following the full moon that follows the northern spring equinox (the paschal full moon), as calculated by the tables set down by the First Council of Nicaea. The US Naval Observatory has a webpage that describes how to calculate it. This website does the calculation for you. So Easter in 1429 occurred on March 27th by the Julian Calendar. Count 46 days back and you get February 9th.

I've actually found a note that I made a while ago that suggests 9th Feb- but I'm still no closer to actually knowing how I might easily look up or work out other dates like this- so any help still appreciated! (And any correction, of course).