As far as I am concerned, the best time to do these events is when you have a new catalogue and new product. This allows you to create excitement around the event by saying things such as be the first to experience our new X, it is awesome!
It also allows your customers a reason to come out – there are new things to see. The goal is that they will come see the new X, want it, and book a party to get it free or discounted. You may also get someone who comes out and loves everything you have, and wants it all but cannot afford it all. You could get a new recruit.
When you do these kinds of events, they are not always all about sales, but you can offer incentives to get the sales if you want. I see these are more of a chance to let people see what you have and get bookings from it.