Like any other normal person, I have been collecting snippets of dates etc from things I read (online, dead-tree format etc) into a spreadsheet.
I have not (yet) raided any “on this day” sites systematically, though they’re linked at the foot of each post.
I also have a couple of big fat “On this Day” books which I might raid at the end of this year for next year’s entries, should there still be gaps.
I may even put a call out for people to send me snippets – who knows.