Initially there was a concept of salary to be given on weekly basis to the workers in India. But when Britishers started ruling India they came up with concept of Monthly salary.
Therefore, if we consider 4 weeks in a month, 13 salaries should be distributed and as per monthly structure they were giving only 12 in a year.
In later days people realized it that its a loss to them in terms of one salary. There were many rallies in oppose of this in Maharashtra during 1930-1940. To come up with a solution to this problem, Britishers had a discussion with leaders about how to distribute this 13th Salary and unanimously they decided as per Indian tradition, Diwali is the biggest festival of India, so they will distribute this 13th Salary durring Diwali, today known as Diwali Bonus.
This was implemented from 30th June 1940.
Today many people don't know this, hence, sharing this letter.
http://ccggoo.blogspot.in/2015/10/bonus-brief.html