Reference - Word of Mouth
Verification - Yet not verified, how true this is.
Initially, in India, salary was paid weekly.
When English rule came, they stopped this practice and started monthly salaries.
After few years, Indians felt that something is wrong. They realised that - Monthly salary system is in profit of Englishman and loss of Indians.
They gave the logic that there are 52 weeks in a year. In one month there are 4 weeks. Hence 52 divided by 4 gives 13. If weekly salary system was prevalent, they could had got 13 salaries. Whereas the Englishman is cleverly paying only 12 salaries for 12 months.
This results to one salary loss of Indians and 1 salary cost saving by Englishmen.
Accepting this as a logical argument, Englanders agreed to pay the 13th salary. It was agreed that this salary will be paid at time of Diwali. Hence from that day onwards Diwali Bonus became an inseparable and legal part of Indian economy.