When we are entering sermons, we have tried to identify dates for specified occasions when they are not given in the original manuscript. We have used the following sources to do so.
Calendar of saints (Church of England) (this entry also discusses moveable dates from the CofE calendar)
Cheney, C. R. and Michael Jones, eds. Handbook of Dates for Students of British History. 2nd Edition. Cambridge, UK: Cambridge University, 2000.
Frequency of Easter Sundays (finds the date for Easter for both the Gregorian and Julian calendars)
Universal Calendar (finds calendars for any year and country)