31st March is important in many ways. Firstly, it is the date on which the Banks finalize the accounts. Government and many private and public organisations also finalize the accounts on the same date. The budget of the next year is to be approved by the Parliament before the start of the next financial year ie. April to March.
As far as Postal Department is concerned, this is the day on which the first Post Office in India was opened in Calcutta in 1774. The postal services was only for the government earlier. It was from 1774 onwards that it was opened for the public, though limitedly.
There will many important events connected with 31st March. It is, anyway, an important date.