The United Nations has declared July 11th to be World Population Day:

World Population Day, which seeks to focus attention on the urgency and importance of population issues, was established by the then-Governing Council of the United Nations Development Programme in 1989, an outgrowth of the interest generated by the Day of Five Billion, which was observed on 11 July 1987.

You might be interested in The Best Resources For Learning About Our World’s Population Of 7 Billion.