The most obvious and flexible solution would be to store all the events and its beginning and ending "show"-dates in a database and SELECT them through a suitable SQL-command. If I'm not mistaken, you can USE DBI in the Template Toolkit; so the interface to the database would be already there.

As the SQL of the most popular databases sport some kind of date or now function you would not even have to code that in your template, it could all be done in the database itself.