I think I've finally got my "On this day" feed functioning after following Ben's advice.
It looks like there were some other issues which caused the feed to become invalid based on the way the post content was interpreted. These lead to the creation of "unrecognised elements" in the item descriptions, thus breaking the feed, but only happened on certain posts. These should now be resolved so, fingers crossed, it'll be working properly.
I'll have to wait until tomorrow to find out for sure.