Christmas comes but once a year.
“Alle Tage ist kein Sonntag.” ← The German sentence,
The translation is, “Christmas comes but once a year.”
… Is this a clownishly bad mistake or a strange colloquial thing I have not heard before?
It’s a legit translation of a proverb, or, in this case, an old German song. https://www.volksliederarchiv.de/alle-tag-ist-kein-sonntag/
Sonntag, sunday, is used in many metaphors associated with slacking of. When used with activities, it is often pejorative, as in: you seem to be only doing this in your free time. cf. Sonntagsfahrer for a bad driver.