During his rest break in Post-Op, B.J. addresses Igor as "Corporal", but later, in "Promotion Commotion", we see Igor is an E-2 ("buck") private.
Klinger: Fantastic! You know, Major Cass, those are the kind of ideas that keep the sun from setting on your umpire.
Maj. Cass: Quite.
Klinger: When you get your next word typed, let me know. I'll be in my office. Hold all my calls unless it's MacArthur.
Col. Potter: Damn!
Klinger: (from inside the office) Seven!
Winchester: You know what a rum baba is?
Pernelli: Yes, it is one of them Desi Arnaz songs.
Winchester: And you call yourself a chef?
Pernelli: No, I call myself a cook. Chefs ain't got tattoos.
Col. Potter: (typing) One. O-N-L-(grunts). Only one. Y-space-O-P! Damn!
Klinger: Pretty good typing. You're up to 6 'damns' a minute.
This is the only "post-Christmas" episode of M*A*S*H.
The title is a reference to the poem 'The Night Before Christmas' by Clement Clarke Moore.