It might not be Christmas (in fact it is mid March whilst I'm writing this!) however lots of people are already planning their Christmas party. Only a few weeks ago I actually performed for a late corporate Christmas party - they seem to be held any time these days!
Having a mentalist at your Christmas event can help make it memorable and bring people together. Usually at this type of event you'll find close up or walk around magic / mentalism is ideal. Maybe you are planning a drinks reception before a meal, or just drinks at a bar or other venue. Keeping people entertained is what makes your event a success and mentalism is one of the best ways to do it.
One event I performed at before Christmas was at a lovely hotel on board a yacht in London. A group of maybe 50 people. I started performing as people arrived, which is an ideal time because this is before groups are really getting together and people are more likely to be on their own or in small groups feeling a little awkward.
Some things to consider are noise. For walk around magic it is best is music is not played at full volume. Performing in noisy venues can be a challenge, both as a performer and also for the audience trying to hear what is being said. That is why performing near the start of an event is often great when there are less people and therefore less noise.
Think about booking a magician for your next Christmas event and make it something people remember.