Well... I answered my own question with a touch of research. Di is used in some particular grammatical constructions and di notte, di sera, d'estate, seem to be just a few. Here's the site I found the information on, rather handy! http://italian.about.com/library/fare/blfare157a.htm
In the Occupation chapter of Italian Duolingo, I've found that the particle "di" can also mean "in the". E.g: Il postino lavora di mattina--the English translation said that "The postman works IN THE morning".
So, which one is correct? Or, is this case just an exception?