World Humanist Day is indeed an international holiday. Various groups celebrated a Humanist Day at different times until the International Humanist and Ethical Union (to which many established groups such as the American Humanist Association belong) settled it on June 21. Most years, this is the summer solstice; in 2012 the solstice arrives late on June 20,thanks to the leap year correction.
The IHEU notes the solstice connection, but doesn’t say why the date was chosen. Their website suggests a picnic on that day, which suggests that the organization has a northern hemisphere bias: people in Australia or South Africa might find that suggestion inappropriate due to weather and the early dark.
The idea “humanism” has been around at least since Auguste Comte (1798-1857) wrote about a “religion of humanity”, but in America, at least, the term was not in wide use until the 20th Century. Before that most people could not imagine ethics apart from a creed or commandments to support it. Arguments over doctrine were intense; hence the term “freethinker” was in wide use. Freethinkers, however, had a range of ethical stances.
The IHEU defines humanism in this way:
Humanism is a democratic and ethical life stance, which affirms that human beings have the right and responsibility to give meaning and shape to their own lives. It stands for the building of a more humane society through an ethic based on human and other natural values in the spirit of reason and free inquiry through human capabilities. It is not theistic, and it does not accept supernatural views of reality.
Until well into the 20th Century popular ideas of ethics were drawn largely from the Bible and were focused on the individual. Some things were agreed upon (e.g. killing is wrong, you should help your neighbor) while others (slavery and women’s rights) were matters of great debate.
Robert Ingersoll was one of many we would now call humanists. He gave a lecture entitled “Liberty of Man, Woman and Child,” which expresses his view on personhood. And his “creed,” as he said in slightly different language on several occasions, was:
Happiness is the only good. The place to be happy is here, the time to be happy is now, and the way to be happy is to make others so.
With a view of the bay from my window, I think I’ll skip the picnic. I will enjoy the long day, relish the early morning return of the light, and get on with the business of seeking to make the world a happier place through writing.
[Robert Ingersoll plays an important role in the life of John Emerson Roberts. See Books page.]
freethoughtandmetaphor 
Jun 20, 2012 @ 15:21:28
Happy World Humanist Day to you.
We live in a zoo.
We look like monkeys
And we smell like them too
May we live in a world where individuals and groups who are not ” theistic”, and don’t accept supernatural views of reality are able to live openly, make contributions to civilization, and be judged for citizenship, rather than espousal of formulaic creed.
May we live in a world where the 90 plus percentage of humanity (including myself) who believe in God are able to live openly, contribute to civilization, and be judged for our citizenship, rather than allegiance to formulaic creeds.
There’s more to “supernatural views” then the intolerance, murder, and mayhem too often perpetrated by the adherents of these creeds. The French Revolution and Soviet Communism both tried to eliminate religion, and failed. It’s best when tolerance goes both ways
“Love thy neighbor as thyself” is great citizenship.
Jun 20, 2012 @ 16:13:05
I agree. Tolerance should go both ways. Thanks for commenting.
Jun 21, 2012 @ 12:07:47
I joined the American Humanist Association when I was a freshman at Harvard (1947-1948). I have never been an active member, but I have been at two conventions. The organization has had its ups and downs, with turmoil in the late sixties and seventies that resulted in the departure of Paul Kurtz, who founded the Council for Secular Humanism, an organization with which he has recently broken because it was not doing things his way. In its turbulent period the organization became excessively politically correct, a sin the Council for Secular Humanism has never been accused of. During its politically correct period it ran an article that, among other things, attacked Lee Baxandall and the Naturist Society complete with quotes from Nikki Craft. Lee wrote a letter in reply to her charges, but it was never published. The AHA has backed away from those excesses, but generally I feel more at home in the Council for Secular Humanism.
Your quote from the founding document of the AHA includes a rejection of supernatural beliefs, but the organization includes members who regard themselves as religious humanists. A distinction I have made in my own mind has been that humanism is essentially a morality based on furthering human well-being, while the ‘secular’ in secular humanism refers to an absence of theistic beliefs. In these terms I am a secular humanist.
Jun 21, 2012 @ 13:58:20
Thanks for your comments, Dick. My sense from the website is that the IHEU is down on the term “religious humanist” because they think the Unitarians have laid claim to it. They don’t want to see the term “humanist” modified by anything. You and I are both humanists.