What is a loan maturity date?

The maturity date is the date by which the principal and interest must be repaid in full to the lender.

What happens at and after the maturity date?

Once the maturity date is reached, the monthly payment is no longer needed as the debt amount has ready been paid in full.

In what document maturity date is specified?

It is specified in the mortgage note (promissory note) document.

Why maturity date is necessary? 

It is necessary, so the lender is assured that their money will be paid back within the expected time period.

How it is calculated?

If the margate note states that the payments must be made monthly, then the maturity date is calculated in months. The date is determined based on the timeframe the loan needs to be repaid. If it's 30 months mortgage, then the maturity date is calculated from the first month date plus 30 months period. So if loan starts at April 1, 2022, then maturity date will be on March 1, 2052.