Contest at Classic Castle can be both at a specific time of year and not. Here is a link to information about past contests.http://www.classic-castle.com/events/events.html
As far as being alerted to them in advance, well advance notice is only offered when and if the people sponsoring the contest give that information. Most often advance notice is not given. In addition, most contests that are run from year to year do not have the same categories or even the same building content.
So the answer to your question is to be patient and wait for an announcement to happen.