Midsummer's Eve is one of four nights in the year where all the creatures (except the humans & brownies) run wild, due to the fact that almost all of the boundaries are lowered. The year that Kendra and Seth came to the Fablehaven the event was on June 20, which was also the day before the summer solstice. The only limits to where any creature can roam and work mischief were the walls of the house; which unless invited into, they cannot enter. In Fablehaven, Seth and Kendra are told on this evening not to look out of the window for their own safety. But, Seth actually opens the window, letting a whole horde of evil mysterious creatures in, setting up the plot of the majority of the book. Seth, who realizes his mistake, brings Kendra into the salt and iron filing circle around his bed. He later uses the mixture as a weapon.When he lets the creatures in the window in the form of a baby. The baby when it came into the window turned into a goblin. Later in the plot the creatures outside cry out "Seth, come let us in please Kendra!"
There are three other nights where the same thing happens. They are the winter solstice, and the two quinoxes.