Add support for summer exams

[kashav]

So when I originally developed this, I kind of forgot about summer exams (oops).

Right now, if any month other than 'apr' is provided, I default to 'dec'. Won't be hard to remove this caveat, but it'll also need to be tested with summer months.

Also, I think it'll be cool if we chose the default month based on what's closest. Let's see if we can get that implemented as well.