No idea; I tried looking fori it, even tried to use korean transcription for monday but i had no results whatsoever.
I can't remember the reference in either of the dramas - I dropped both of them and perhaps it's also an incorrect translation or maybe something entirely different altogether.
The best thing I can suggest is to look through, for instance dramabeans for recaps and detailed analysis - maybe the author of those detailed drama anlysis will mention this.
Or hopefully someone else on the forums will be able to be more informative and helpful.
