Make the first visit work in your favor!
Going into the dealership on the last day of the month, or close to it, will certainly be a very uncomfortable and aggressive environment if you're not ready for it.
Usually, most car dealerships sell cars in the last half of any given month. It's a no brainer that you can expect the staff to be overwhelmingly busy, and the ones that aren't necessarily busy, are trying aggressively to close a deal.
The best time to go?
THE FIRST FEW DAYS OF THE MONTH!
Set an appointment with the internet and business office that handles calls, tell them you're interested in test driving such and such, and be straight forward with your salesperson about your intentions for that visit.
This not only sets the salesperson up to be less aggressive, it also gives you an opportunity to try and learn about the vehicle you are interested in without the distracting uncomfortable atmosphere.
What if you fell in love with the car?
You most certainly found the WORST time to get the BEST deal...
LEAVE THE DEALERSHIP.
Landing a good deal will work in your favor if you...
Make sure and leave your information with the salesperson as well as take his business card.
Follow up as well with a thank you email (salespeople love that).
Start planning out and researching how much you'd like to pay for the car.
Odds are, your salesperson is likely to be following up with you throughout the month, and if you can, certainly do find a way to present the price you'd like to pay to him (as well as another dealer) before you walk back in.
You're more likely to get the best possible deal on your car when you let your buying emotions calm down after test driving and seeing the car you'd like to buy, and planning out how and when you'd like to make your offer to the salesperson.
(Also, it gives you a chance to shop the price to another dealership if you absolutely could not stand the salesperson you worked with the first time around!)