There are a number of reasons why you may require car insurance for just one day. It could be that you need to borrow a friend's car for a day or arrange short term cover when purchasing a new car. Whatever the reason for needing 1 day insurance, insurance companies are willing to quote for short term cover as well as the more normal annual insurance policies.
One day insurance is obviuously a lot cheaper than buying an annual policy. However, that doesn't mean it will cost the same as an annual policy divided by 365. Factors involved in the cost of the policy will include your driving record (how long you have held a licence and whether you have any driving convictions), your age, your occupation and the make and model of the car. Just like with annual car insurance, the more powerful the car the more expensive the insurance cover will be.
One day insurance is not just limited to cars. Short term deals are available for vans, motorbikes and other types of vehicle too. When looking for this kind of cover it is always worth shopping around as you will find a wide variation in prices.