I think officially you are allowed one a day and it shouldn't take you out of ketosis. I regularly have Diet Coke (I don't like Zero) and stay solidly in ketosis as there are no carbs in it, and for me the citric acid doesn't seem to do anything.
I wouldn't have thought it mattered in terms of ketosis. But I do find that Coke makes me feel hungrier, so you may want to leave it a few days until you are solidly underway.