Do you give the Friday before the 4th of July off? The day after Thanksgiving? Imagine being a new employee recently completing a move to Boston. You head in to work on Patriots' Day to find out you are the only one in the office. Or worse, you come into work on Patriots' Day and only half your team is there. Do you get angry that nothing gets done? Do you give the staff that showed up an extra day off?
Employees want to know in advance so they can plan that bbq or ski trip. And they'll remember when they work a day when others are 'off'. For your part, if an employee actually asks, you don't want to have to think about days off on the fly - especially when your answer is no. Worse yet, you don't want to get irked when 3 of your team don't show up on President's Day because they thought it was a Holiday.
In the US, on average, companies usually provide 10 paid Holidays.
Source: 5USC Sec. 6103.
If you want to give more days, fine. Give 'em. At a moment's whim you can still declare National Administrative Professionals Day or International Talk Like a Pirate Day as a paid day off. You can decide at the bar that you'll shut down if there is enough snow to go sledding. It will even feel like a cooler place when staff get days beyond what was expected. But you'll also be laying the foundation of a strong company (think audit and legal), giving yourself a starting point to address problems, and employees want to know.