Are mandatory meetings on a day off for a contractor worker legal?

It probably depends on the terms of the contract. Contact an attorney if needed.

Are you an employee or a 1099? If an employee, they need to pay you for working over 40 hours in any given week overtime. However, don't think there is any law that prohibits you from being forced to come in on a legal holiday.

Keep all your documents (emails, memos) requiring that you attend. If they are controlling you by mandating that you attend their Christmas party, you may be an employee, not a contractor, and they may be required to pay you on payroll and pay you benefits, etc. This could be a classification issue. The one thing in classification is that an employer cannot control an independent contractor. For an employee, the employer can make a meeting mandatory, but if this is your scheduled day off, you may be entitled to compensation. And again, this control could indicate your "employee" status (which could be good or bad for you, since contractors do not get any benefits but do end up earning more money sometimes than employees).