Resignation Letter

So, I just received an offer from a Big 4 company (I work at a smaller regional firm now), but my start date is not until October 3, 2016. I am trying to determine the appropriate time to notify my current firm that I will be leaving. Should I let them know soon (like next week) that I will be leaving at the end of September or should I just give them the standard two week notice in the middle of September? I am leaning towards letting them know sometime around the end of August, but I appreciate any feedback that anyone has.