I operate a Service Bureau, which is a firm that facilitates transactions for brokers and banks. There are two calendars for US Equity Trading that we need to maintain: Trading calendar and Settlement calendar. The difference is that trading happens at exchanges/brokers and settlement happens at banks. Thus, a bank holiday may be trading day -- for example Veteran's Day. We're small, so I personally manage these two calendars (and as others say, we use DB tables).

There's a settlement concept called T+2 (soon to be T+1) which is Trade date plus 2 days; this is the day when the cash and shares must be exchanged.

On the Friday morning before last Veteran's Day, we get operations emails from one of the Banks we service that our Settlement dates for the previous day's trades are all incorrect and causing issues for them. I was very confused and very worried that I somehow forgot Veteran's Day.

But... then I realized that it was a Canadian Bank settling US Equities trades!

In their business practice they would settle the trades on a day all the US Banks would not. T+2 for the Canadian Bank was different than T+2 for US Banks. So we resubmitted all the trades with a settlement day of Veteran's Day.

As OP says, managing business calendars is hard for many reasons. I also got really jacked up by a Leap Second, a story for another day.