Giving in Q4 - fact or fiction?

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Go ahead, ask anyone who gets snail mail. Or anyone in fundraising. Or anyone in estate planning, wealth management, or tax advising. They'll tell you, "The fourth quarter is the busiest time for charitable giving." Geez, if you'd asked me I would have told you this also. Just as we "know" that retailers turn profitable for the year on "Black Friday," we all know that most giving happens in the fourth quarter, during Giving Season, right? Well, wrong. Or, rather, maybe we don't know what we think we know.
I went looking for the stats. I wanted to compare Q4 giving in the US to Q4 retail spending ($474 bb, National Retail Federation) or Q3 airline profits ($1.6 b net income for the top 10 US airlines, Dallas Morning News) or the cumulative value of 2007 mortgage write-offs for major banks ($60-70 bb, Financial Times).
So I have all kinds of things to compare the Q4 giving number to, but, alas, I have no number. I've checked AFP, Foundation Center, Giving USA, CoF, and made some calls - I can't find a quarterly breakdown of giving.
I suppose I could create my own index and start computing a representative number...or I could divide the $300bb in 2006 by 4 ($75 b) (but that assumes giving is equal across all quarters) so I'd have something to compare to the numbers above.
Help me out folks - are there numbers, over time, to document whether Q4 is the busiest time for giving?
Tags: philanthropy, socialcapitalmarkets, giving, givingseason