End of Month
Each salesperson at a dealership is encouraged to meet certain sales quotas. These quotas are tallied on a monthly basis, and if a salesperson hasn't managed to make the number of deals necessary, he will be hungrier to sell than you are to buy. After all, you can wait until next month before buying a car. The salesperson will never get the same chance for that bonus back if he lets it slip away. Nonetheless the experienced dealer can be a tough nut to crack. Some will simply negotiate a fair value to close the deal; others will try to haggle for as much money as possible. Like everything else, dealers come in all types. Also keep in mind that the end of the month is not a great time to shop for most people. That's when a number of bills or payments come due.
End of Model Year
New car models are introduced as early as late summer and can continue on till winter. This is an excellent time to get better deals because dealerships are trying to be rid of the leftovers, giving you room to negotiate. The downsidewhich you should mention when negotiatingis that you're buying a car that already has a year's worth of depreciation on it. If you want to sell the car two years from then, it will be evaluated as a three year old car. Another downside is you may only find a very limited selection. You might not be able to choose the options you were hoping to get, or you may get a car with too many options you don't want. Also, if a particular car model has too many leftovers, you should probably take into consideration why.
End of Calendar Year
So basically December. At the end of the calendar year is a time when some models reach the end of their model year, and automakers are likely to be looking to close the year with as many sales as possible. Winter months are the slowest season out of the year, and dealers and automakers alike will be trying to get as much out of it as they can.
Rebates and Incentives
Holidays and long weekends will frequently see rebates, incentive or cheap financing offered. Special promotions occur all year round, and you may not want to wait till the end of month/model year/calendar year if a car you want is going hot.