I think they mean when it is peak times or close to it. Summertime, Spring Break, and Christmas Time are the seasons. We go to Disney mainly in the off season, so those restaurants are never opened.
There's no designated times. That means they may open whenever Disney thinks they need extra restaurant space, usually in the most busy times of year. It's not always holidays. They've opened some of these places in September to accommodate influxes of free dining. Of course that doesn't mean they'll open EVERY September, or even every day in September.