Overlap in employment of 1 day

What is this relieving and joining?  Relieving from where and joining where else? Please explain the full story.

Relieving from a Company as an employee and joining another Company as an employee. 

You cannot get relieved at afternoon from a company and join at forenoon of the same day in another company.

Your last workig day with the previous company cannot be the first working day with the new employer.

You will be liable to give explanation for this lapse to the new employer in cse it is found in the background verification. 

So better inform the new employer about this in your joining report so that you are not held to have committed an offence for  breach of trust or cheating with the new employer due to this lapse. 

Is there a law which prohibits this overlap?

My relieving letter shows this clearly so. What can be solution to this? 

You will be considered to have cheated the company by not revealing that you are still employed by the previous company as on the date of joining the present employer, therefore you can request either of the employer to either accept your joining one day later or to relieve you one day prior and to adjust the settlement accordingly, if their law permits to do so.

This is a trivial matter, which should not be blown up.