St Crispin's Day was the day that the Battle of Agincourt was fought, so R-M was probably trying to make some tedious point about English exceptionalism. Probably sees himself as the spiritual heir to Henry V, or some such crap.
no, st crispin’s day is the 25th October, it’s a catholic feast.
I mean I don’t know if that is the usual way he dates his letters but it’s certainly not an invalid way of doing it, just not really usual these days.