Maybe he had more than one party.
No, he specifically said it was the party for his clients.
Or more likely, it took months to find anyone that would attend.
This, I can believe.

Oh, Louis, will you ever win?
Well, it gets more complicated if you count the animated series. I used to think the second movie was incompatible with
The Real Ghostbusters because of the lawsuit/shutdown element, but then I came across a rather clever
fan chronology postulating that the lawsuit actually "meandered in court" for some three years before any legal action was taken.
Clever, but that would have to be really forced to make it work fully. In the cartoon, if I remember correctly, the GB were superstars and pretty much loved by all. Also, the supernatural was pretty common place and easily believed. In GB2, the implication was that the GB were frauds and were properly shut down and were, more or less, disgraced and considered has-beens.
Also, legally speaking, if they were sued by every "state, county, and city agency in New York", I highly doubt they would have even been allowed to continue business until the cases were settled.
Things to note: When Dana is having her interview with Egon, Ray, and Peter she says that she hasn't been back to her apartment in three days. So at least three days pass between when Dana has her experience in her apartment and when she walks into the firehouse.
We've to to assume at least a full day passes between this and their first job at the Hotel Sedgwick since it'd take at least that long to get Ecto-1 up and running and in prime condition -however the boys DO toast to their first customer -Dana- before going on their first call and it only makes sense to do this if this is occuring later that same day. So maybe Ray put some hustle in on Ecto-1 between the interview with Dana and that night?
We've got to assume it took atleast a full day to get the loan(s) approved, find the firehouse, and "move in." Probably at least a full week would've had to pass for them to get all of the equipment built, esp. the containment unit, and Ray's "relax, I found the car" suggest maybe they'd been looking for one for a while.
Given Peter's non reaction to Ray's declaration about the car, it is implied that only Ray was worried about it (which is pure Ray

).
Also, Dana specifically says
two days that she hadn't been back to her apartment.
Lets see, in the film it goes like this:
1) They agree to buy the firehouse.
2) Dana has the fridge experience
--Two days pass, as explained by Dana--
3) Ray gets the car/Dana visits the GB/Peter goes to Dana's apartment
Since they really fixed up the fire house, were able to interview and hire a secretary, and were able to film and get a commercial to air, I would argue that at least a week and a half to two weeks passed between
1 and
2.
Presumably, the "petty cash" meal takes place the evening of Dana's visit to the GB (the shooting script confirms it). If that is the same day as
3, then Ray got that car fixed up pretty damn fast, as it was bought earlier that day.
Something interesting to look at: what they are wearing. When Ray brings the car, he is wearing a blue, jeans-type jacket. Peter is wearing a sweater vest with a stripped button down shirt. The next scene (when Dana visits), Peter is in a complete change of clothes. Ray, however, seems to be wearing the same thing (you can argue, possibly, that he's merely wearing work clothes). So, it is possible, just possible that there was a time gap between when Ray bought the car and when Dana visited.
Yet, that still leaves us with Ray working on the clearly unfinished car earlier in the day, yet it being completely finished by that evening. How about this possibility: Ray fixed up the car and got it painted between the few days (two at most), but he was able to find
another car to pull some spare and extra parts.
The other thought is that, even though it is implied otherwise, the hotel encounter takes place on a later day. Evidence for this is that they are all in a change of clothes, including Janine (and it would be odd for her to change, since she doesn't live in the firehouse).
The montage would've had to take place over the course of weeks for their populairty to rise but this also could've only taken place over one or two weeks -an appropriate amount of time for Egon and Ray to do their research, for Louis to plan his party (though a month-long planning wouldn't be out of line.)
I would think the montage takes place over the period of a month. Considering the rise of their popularity, their appearances on talk shows and numerous publications, and the growing concern by the EPA. A month would seem long enough, but not too long.
All and all, I'd say the move takes place over the course of a month between Egon, Ray, and Peter getting kicked off the university through the battle with Zull.
I would argue the whole movie takes place over the period of 3 months.
And I don't, personally, accept that cartoon series as "canon" (infact the cartoon itself suggests that the movies do not occur in their universe only that the movie we saw was -losely- based on the Ghostbusters' first job) and, obviously, the second movie takes place five years later and itself likely only over the course of a few weeks.
The events of the movies do happen in the cartoon, just slightly differently (especially in GB2's case).