For most people, every day is an internet day, so the term is useless. However, when you cannot afford a monthly internet subscription, and you have to buy internet on a day by day basis, the term takes on a whole new meaning.
Flatmate 2: No?
Flatmate 1: Google it.
Flatmate 2: It's not an internet day...
Flatmate 1: Internet Day?
Flatmate 2: INTERNET DAY, YES MATE.