We've written about ACABAR de before -- because it's such a useful combination. When you place a de after acabar, it takes on the meaning of, to have just. ACABAR in Portuguese by itself means to end, to end-up, to finish. I've noticed that so many people are getting these two forms mixed-up (confundida).
Examples of acabar in Portuguese
A few examples will help clear this up:
Examples of acabar + de
That de makes a big difference.