Just happened on this obsolete term in OED (most recent cite is 1425). It’s defined as the night before Sunday, ie Saturday night. It had some companions too:
Tuesnight, Wednesnight, Frinight and Saturnight, all on the same principle. Notice the gap and say hi to Thurseven, the odd one out for some reason, but then as we all know here, languages could never abide strait-jackets.
I like them, although there’s certainly room for confusion if they were ever revived. But then our ancestors managed and were they any smarter than us?
BTW just noticed that there’s no Monnight. Go figure.