Do I have to pay an employee for an overnight seminar?

You say the employee is salaried. If she is a salaried employee, you can essentially have her work any number of hours without additional pay.

The one potential wrinkle would be if she is a salaried employee who is not exempt from overtime; note that simply being paid via a salary does not, in and of itself, make an employee exempt from overtime. To be exempt, an employee must be paid a salary *and* his/her job must meet certain criteria. You can find the criteria on the Dept. of Labor website, under "wages and hours" (then look under "overtime," then under "exemptions"). Usually, it is managerial, upper-level administrative, and professional staff who are exempt.

If she is salaried and exempt, you have no problems--she doesn't get extra money under any circumstances. If she is salaried and not exempt, you have to pay her extra if she works more than 40 hours in a week. However, you don't necessarily have to count time after the seminar as work time, at least if you don't have her doing any work after it (e.g. she's free to go out to a movie, dinner, just read a book, etc.)--and especially if she does have the option of returning home. You may still owe her additional compensation if the seminar time, plus the extra work she did that week, itself pushes her over the 40 hour mark, if she is not exempt.