I purchased a used car Saturday and want to know if I can cancel the sale the following Monday

From a dealer or an individual? Why do you want to return it?

Typically, you can't just change your mind. A lot of people have misconceptions in regards to their "3-day right to cancel."

Like Judy asked, why do you want to return it? Did you buy it from a private party or from a dealership?

If you have only paid a deposit on the car then yes you can cancel, but you will lose your deposit.

Unless there is something mechanically wrong with the car then I doubt they would take the car back.

Some places won't even take a car back if it is faulty as they say that you might have done the damage and the car was fine when it left.
Always get a warranty, from a dealer even if it is only a one-week warranty.

This happened to my partner, he purchased a car from a dealer, the engine blew up 2 days later through no fault of his. They would not refund the car.