What is "settling day"

Settling \Set"tling\, n.

1. The act of one who, or that which, settles; the act of establishing one's self, of colonizing, subsiding, adjusting, etc.

2. pl. That which settles at the bottom of a liquid; lees; dregs; sediment.
   --Milton.

   Settling day, a day for settling accounts, as in the stock market.