Pity they didn't explain the meaning of the holiday to you in school. It has a lot to do with Ivan Vazov that you're currently reading. 
It's not so much of "enlightenment" but of people who enlighten others - bring light; wake up them. Bulgarians have managed to keep national identity, culture and religion under five hundred years of Turkish dominance. And it was not a civil dominance. 
You have a vocational holiday today because you are a teacher, you open the eyes of kids - show them new things, bring many and different possibilities for development or at least that should be the higher meaning of any teaching.