I personally don't think he was planning on attacking Katie on that evening specifically. But I don't have a particular reason for that, it's just a gut feeling and obviously impossible to know either way.
One thing to note (that may or may not relate to the above) is that Brian and Katie lived in an apartment. At that point, EAR/ONS as far as I know had never attacked anyone in an apartment, although he did shortly afterward.
I don't think that has anything to do with whether he was planning on attacking Katie that particular evening, but what I realized it very well COULD relate to is why he would be stalking and calling Katie at work (as opposed to at home, which is what EAR/ONS usually did)--he probably couldn't access their apartment or their mail to figure out who they were.
As far as I know though it's possible that he could access their mail/apartment, I have no knowledge of that either way. I'm just speculating on account of mail, and in many cases the residences themselves, being harder to access at apartments than at standalone homes.