Boo! at the Barns 2014 is taking place on Saturday October 25 from 5 p.m. to 10 p.m.
In my experience, when an annual event survives the 3-year milestone, it's proof that it's becoming a family tradition. Boo! at the Barns is in its 5th year at Wychwood Barns. I've not attended it yet but I will visit it this Saturday. They say it's their largest event of the year. Admission is free but there will be charges for some activities. (Note from October 29: At the end of this page, I've just posted a photo gallery of the event (which I attended with a little pirate and his mom, also a pirate, it runs in the family).
By the way, while you're there, you might want to do a little stroll along one of the few private streets in Toronto. It is truly lovely and part of the Wychwood Country Stroll (walk #21) in my walking guide Toronto Urban Strolls 2.
Note that Wychwood Barns Park also does a Pumpkin Parade on November 1st at dusk, an event where locals bring their carved pumpkin lit with a candle, to line paths and for everyone to enjoy.