1. No federal or state law requires that you be given vacation leave. Of course, most employers offer paid vacation days anyway to be competitive and attract good employees - but there's a difference between what's smart and customary, and what's legal.
2. No federal law requires that you be given sick leave. A very small number of jurisdictions require paid sick leave, but the majority of Americans live in places not covered by those laws. (The Family and Medical Leave Act, or FMLA, does require some employers to give some workers up to 12 weeks off per year for serious illness or to care for an ill family member or new child, but that time off is unpaid.) Of course, as with vacation time, smart employers offer sick leave anyway to attract and retain good employees, but some employers don't. Some entire industries, such as restaurant work, are known for not offering it.
3. If your office closes for the day because of weather, a natural disaster or even just because your employers decided to give everyone the day off, they can deduct that time from your leave balance – even though it wasn't your choice. Alternately, if you're non-exempt, they can dock your pay for the day. What does non-exempt mean, you ask? The federal government divides all types of jobs into one of two categories: exempt and non-exempt. If your job is categorized as non-exempt, your employer must pay you overtime at time and a half your regular pay rate for all hours you work above 40 in any given week, but your employer can also dock your pay when you don't work at least 40 hours. If you're exempt, your employer isn't required to pay you overtime but also cannot dock your pay, except in limited circumstances.
4. No law requires your office to leave you alone while you're on vacation. Because no law requires employers to give paid vacation time at all, employers can structure the time-off they offer however they like: They can say that you can take the time as long you answer your cell phone, or as long as you check email once a day, or as long as you get that report done while you're away. So yes, you might be on a cruise or hiking in the mountains or just relaxing at home and officially not working, and your employer can still bother you with work calls and emails. A smart employer won't do this because they recognize that having time to truly disconnect is important, but many others do anyway. (Of course, a smart employee might be conveniently "out of cell phone range.") If you're non-exempt, you would need to be paid for any time you spend working on your vacation.
5. Your employer can revoke its approval for your time off at any point. No law prevents your boss from changing her mind and denying your vacation time, even after it's already been approved, and even if you have nonrefundable airline tickets or are already on the beach sipping a mimosa.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.