Look up NYSE rule 7.2 - a public holiday that occurs on a Saturday is typically observed on the Friday prior to the holiday, except in cases where that would interfere with quarter-ends and other unusual circumstances. This is why the New Years Day observance is sometimes skipped - December 31 is a quarter-end.

They also avoid being closed more than 3 consecutive days, which is why they are open for trading on Black Friday (but they do close early!).