Literally it means "money (geld) day (tag)". It means the day that the money is due. And if you don't pay, then you are declared bankrupt. That is what i can make out from the text Steven posted. The Geldstagbehörde seems to be the bankruptcy ? authority.
I know absolutely nothing about Swiss law, but in many regions of Western Europe, there was a gathering of some kind on the day that tithes and taxes were due. Usually also the day that the conscription lottery took place. It might have something to do with that.
So if you want to find out,just contact your Swiss banker