You cannot get car insurance for just 1 day whilst being a provisional driver. The minimum cover that you'll get is for 4 weeks.
Most people ask if they can get insurance on their actual driving test but you surprised me by asking for a policy to cover you 1 day before your test. Are you not learning to drive with an instructor? If yes, then why not get 1 last driving lesson with him? If you're learning privately (with parents, family or friends), then you obviously must be insured? Can you explain your situation a bit more?
