To add to that: In several jurisdictions, the calendars of when shares can trade (T, with a closing price) and when shares can settle (T+2) is distinct (say, shares can trade on NYE, but not settle). So, one definitely needs to maintain (potentially) multiple calendars per jurisdiction. One calendar per counterparty was news to me, but fair enough.
The next complication then is that these calendars can change. In Hong Kong, we have typhoons quite occasionally between around August and October, and if the observatory declares a T8 or above, several businesses close, including the stock exchange. It then depends on the exact timing of the raising and lowering of the signal whether there is the usual trading, or half day, or no trading, which implies no closing price, thus an additional holiday for the trading calendar.
To treat holidays properly, one must then take into account the day it was declared for some purposes.
The next complication then is that these calendars can change. In Hong Kong, we have typhoons quite occasionally between around August and October, and if the observatory declares a T8 or above, several businesses close, including the stock exchange. It then depends on the exact timing of the raising and lowering of the signal whether there is the usual trading, or half day, or no trading, which implies no closing price, thus an additional holiday for the trading calendar.
To treat holidays properly, one must then take into account the day it was declared for some purposes.