Actually holidays are not so easy to take into account.

First of all holidays vary by nation. And by state within nations.

Secondly holidays change periodically for various reasons. Therefore definitive lists tend not to stay very definitive.

Thirdly what happens with holidays that land on weekends? Typically different companies will handle these differently. In financial markets there is no hard and fast rule, and the decision about what to do often comes within 12 months of the actual holiday.

Fourth there are a variety of marginal holidays that are handled differently. A given day may be a holiday in the bond markets but not in the stock market. Or vice versa. Thanksgiving Friday in the US is officially not a holiday, but at many places is. And so on.

Fifth there is the question of religious holidays. For instance NYU medical school (which has a huge Jewish influence) routinely takes various Jewish holidays off but did not take off Easter. However most places in the US take off Easter but not the Jewish holidays.

And so on. The closer you look at it, the messier the idea of a "business day" becomes...