China has the largest population and India come after, but according to the prediction, India will pass China in the future and becomes the country with the largest population. Every country will do the census in the certain time, in the purpose of knowing the basic situation and making better plans for the citizens.
Take America for example. It does the census every ten years. As it has many immigrants, so the population changes greatly in the last decade. The latest census showed that the white children became less and the Latin and Hispanic had the large birth rates, so it had been predicted that white children would become minority group in the future for America. As a result, the government has taken action to increase the local people to have children.
In China, thanks to the family plan, the large birth rate has been controlled and most people's life standard have been improved. Now the census predicts that China will face the aging time, so the government has changed the policy, inspiring people to have second child. The meaning of World Population Day is to remind the public of the importance of knowing the changes of the population.