holiday?  Holy Day.
Good?  God.
Invent a new language, goofball.  Leave the “traditional” one to us theists.

The group defines humanism as “a progressive philosophy of life that, without theism, affirms our responsibility to lead ethical lives of value to self and humanity.”

Why?

If there is no God, then why is there ethics or morality?  The only morality is “survival of the fittest.”
Now, it can be argued that societies that evolve morality beyond the personal survive as a whole longer than societies that do not.  However, we must ask where the definition of morality came from.  What is the source of our moral lives?  As C.S. Lewis (among others) so clearly showed in The Abolition of Man, the definition of moral actions varies very little from society to society.  I would submit that the source of this universalism in morality is our design and that implies a designer.

YBIC,
Phil Snyder

“Why believe in a god? Just be good for goodness’ sake.”

Taking this to what I think is its logical conclusion, it should more correctly be written:

“Why believe in a god? Just be good.  BECAUSE WE SAID SO!”

In addition to what Matt said, many atheists and humanists are reacting against what they understand the “traditional” Christian message to be:

“Why be good?  Out of abject fear that if you aren’t good, then God—the ultimate source of all goodness—will torture you for all eternity in the fires of Hell.”

That’s a pretty questionable foundation for morality.  It’s not really what the church ought to be proclaiming, but there have been enough people preaching exactly that message from the pulpit enough times that the message is out there, and it’s the impression many, many people have of Christianity.  Compared to that, secular humanism looks pretty good.

Matt:  The current divisive issues like embryonic stems cell harvesting, euthanasia, abortion and many others you may think of can never find a common agreement as to what is ethical or normative.  At least when it comes to Thou shall not kill some of society are clearly grounded in what is good and evil.

Ross:  Business contracts are more successful in societies that believe in a hell.

Sorry, guys.  You will need to define good and goodness.  Otherwise, I’ll have to believe you believe in whatever you choose for today as good and have not a whit of an idea what tomorrow may bring in your concept of good or goodness.

I think that #8 touched on the point I was trying (inexpertly and incompletely) to make.  Without a common framework for defining/deciding what is “good”, and what “goodness” is, the result of sayings such as “just be good” seems to be a survival-of-the-fittest power struggle over the definition of these terms.
I doubt that this clears it up much, and I apologize for my inability to articulate further.

“Morality is but religion’s handmaiden.” - Bacon

”..there are an awful lot of agnostics, atheists and other types of non-theists who feel a little alone during the holidays because of its association with traditional religion.”

This doesn’t make any sense. The holiday in question would not exist but for it’s association with traditional religion. Let the unbelievers live their convictions or grow thicker skin every December.

But see, tr, they don’t know that the Saturnalia was associated with a god.  Nor any of those other christianized pagan hoidays.  They think there was no religion before christianity.  They also think that goodness has a basis in humanity, like in the atheistic states of the USSR, Yugoslovia, Cambodia, China ....

What is “good”?  Pol Pot thought that it was good to murder 1.7 million people.  Was that “good”?  If not, why not?  Why is your definition of good better than Pol Pot’s definition of good?  Joseph Stalin thought it was good to murder about 43,000,000 people.  Was that “good”?

Some people really think that it was.  Just this past September [2008], the Russians have published a school textbook saying that it was.

Stalin acted ‘entirely rationally’ in executing and imprisoning millions of people in the Gulags, a controversial new Russian teaching manual claims.

Fifty-five years after the Soviet dictator died, the latest guide for teachers to promote patriotism among the Russian young said he did what he did to ensure the country’s modernisation.

The manual, titled A History of Russia, 1900-1945, will form the basis of a new state-approved text book for use in schools next year.

  Source: http://www.dailymail.co.uk/news/worldnews/article-1051871/Stalins-mass-murders-entirely-rational-says-new-Russian-textbook-praising-tyrant.html  Please, click on the link, read the article, look at the pictures.

So, what is “good”? 

Jesus said that there were none good, except God. [Matt. 19:17] 

If there is no God, there is no “good”.

Without ultimate moral judgment, “Be good for goodness sake” tends to degenerate into “Nature, red in tooth and claw.”