1. "x" days expression can be used as an idiom or as reckoning.
2. "x" days "x" nights is always used as reckoning.
3. When sunrise and evening are used. (24 hrs. is meant)
4. The Crucifixion. burial and resurrection "verses" use both (1&2).

Idiom vs reckoning. This is a weak way to try to win an argument. Though it is useful in part.
I thought you wanted to argue 4th day (Wed.) Crucifixion. to 6th day (Fri.) Crucifixion people to show why your position is best.

1.Both sides agree on the Resurrection as being the 1st day of the week.
2.Showing the "Preparation day" of Passover. And the Crucifixion happen on the same day which proves it is "3" days and "3" nights as reckoned. Also many verses can be read to prove this.
3. There is also writings of why a full 3 days was used in the Jews practice of the body remaining in the tomb to allow the appropriate time for the spirit to leave.
4."Then" show when "3" days and "3" nights is used as reckoned and not an idiom.
All of the lower 4 can be pulled from 1 source.[ The Companion Bible E.W.Bullinger 1922]
If all you want is to show ("x" days and "x" nights) as an absolute or forecast, good luck.
I no longer feel confused on the matter of arguing 6th day / 4th day.