I don't think this would work for several reasons. First off, putting time frames is a bit demanding. For example this year, star wars (may the 4th event) started after may the 4th which was clearly unplanned but the event simply wasn't ready. Setting up dates would just create unnecessary pressure on staff to put out the events at that exact time. Secondly, events aren't really planned yearly. That might create even more issues with overlapping with other events like tournaments/classes. And finally, there's already a tradition that certain events happen and when you can expect them (valentines, spring event, may the 4th event, anniversary, butterbeer, Halloween and winter waltz happen)