it's recommended to find out when the fiscal year ends for the car manufacturer whose car you intend to buy (and go the last day of their fiscal year). The deal gets bigger and bigger rebates per unit sold (being applied all the way back to the every unit sold that year). For example, if they hit 100 units sold for the year along the brand line, or if they sell 50 units of a particular model, the manufacturer will pay them an extra $100/unit sold for the entire year. Those numbers may increase by another $50 each unit if they hit another plateau. That's thousands of extra dollars to the dealership that have no commissions that need to be paid. It's pure profit.

What this means to the shopper, the dealer will bargain like crazy to get those last few units sold to get them over the line. I've seen one dealership offer several thousands below their cost just to get the extra thousands of profit that's applied to all car sales for the year.

There are other incentives that the dealership can get too, such as for each warranty sold, or each adder (ceramic paint protection) sold. It's a convoluted nightmare for accountants to keep track of. Quite complex actually.

TL'DR: buy at the end of the car manufacturer's fiscal year. You just might get a screaming deal.