My thoughts are that if it fails on the day, then the car will not have a valid MOT.
Once you take an MOT test, the results are entered in the MOT database. The certificate these days is not valid proof, the database is. If your most recent test shows a fail, then I would suspect that if you drove that car on the road (other than to a test site) you would be driving without a valid MOT.
Cheers
Peter