Having parties for the holidays, the summer, an anniversary or for whatever purpose is commonplace for most businesses. That being said, you need to take extra caution given the company can be held responsible and accountable for anything bad that comes out of the event. For example, serving alcohol almost always immediately places the employer in a position whereby they need to monitor the behavior of those in attendance and even step in to stop someone from driving when/if they are inebriated.
The same holds true for harassment or behavior that would be considered inappropriate for the workplace. In short, have a plan and stick to it or your firm may face some pretty severe consequences.