What is special about Grandma and Grandpa's yard in the novel, Fablehaven?

The house where Grandma and Grandpa live is surrounded by a yard that is protected from most of the magical creatures, except on four specific days of the year. The house is further protected, and only brownies are allowed to enter. The yard includes a pool and a tree house. The tree house is located on the edge of the yard and is where Seth is playing when the fairies attack him. The tree house is fairly large and includes some basic furniture. There is also an over-sized barn on the property which is home to a huge milk cow.