Hostess refuses to cover her coworker's shift on Valentine's Day, so she can spend it with her boyfriend: 'My managers all made it abundantly clear that nobody was allowed to request the day off.'

She was right not to cover her shift. 

When you're working in the service industry, you seldom get to choose your shifts. Schedules come out a couple of weeks ahead of time if you're lucky, giving you very little time to make any plans in advance. But there are times when you know you're going to be working, even when you really don't want to be. If you work at a brunch place, you know you're going to be working extra hard on Easter, and probably won't get to enjoy the morning and afternoon with your family. If you work at Target, you know that Christmas Eve is going to be a zoo, and you won't get home in time to tuck the kids in. If you work in an office and you don't get Columbus Day off, that might be mildly irritating, but it's nothing when compared to the craziness service workers have to deal with during their peak holiday seasons. At least most office workers get Christmas Eve and Christmas off! 

If you work in an industry that requires work on major holidays, you need to plan well in advance if you want to take those days off. I'm talking like a year in advance. If you can get the okay from your boss far before these holidays are even a thought in their mind, you might manage to take them off. But even if you do, your manager might just pretend like they don't remember that agreement and schedule you anyway. That's why it's always important to get things in writing.