They don't post a schedule for Yoga on this website, but I think they have it every day. There is a Yoga Room near the Fitness Center. Check the Yoga Room or the Fitness Center for a current schedule.
The difficulty level will depend upon the skill and experience level of the people attending each individual class. If the class has a bunch of novices or beginners, they will adjust the class to that level. If the class is mostly experienced people, they will adjust the class to that level.
I think they have mats for you to use as people can do these classes on a whim. However, they will let you use your own mat if you have the spare luggage space to bring your own.