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.
Any effort you put out is up to you, but when it is a small gathering, putting out a few treats can be to your advantage.
By having treats, people will stay a bit longer to have a snack and you can set some catalogues out by the seating area. They will browse through them and may find a product that you have that they did not realize and put in an order. They may find an item they want that is more than they can afford, and book a party so that they can get it for free.
Keep the food simple. Treat it as a come and go home party but with two vendors instead of one.
It is always best to get a few items from the new book so that you can show them to your customers. People are visual creatures and if they are able to feel and see a new product, they are more likely to buy it.
It may be advisable to wait a bit and see what other demonstrators say about the product before you make your purchase, especially if you have limited funds to spend. It does not usually take too long for trends to emerge or to discover what is going to be popular.