I'm pretty sure that it is paid in addition to any work that you do on that day (if you work the actual holiday). I didn't work Thanksgiving, but alas I wasn't so lucky with Xmas so I'll be finding out for sure soon.
From the Policy Manual:
Crewmembers with ninety (90) days of employment and who have not called in sick on the day before, day of, or day after a holiday are eligible for holiday pay. Each crewmember will be compensated at a rate of four hours and eighteen minutes (4:18) at their current hourly rate of pay for each holiday. Crewmembers with less than ninety (90) days of employment do not qualify for paid holidays. However, should they work on a holiday, he/she will be paid one and one half (1½) times their hourly wage. Holidays will be paid within the pay period in which it falls with the Company recognizing the following holidays: (previosly listed)