Amazon.com announced this morning announced that the 2008 holiday shopping season "finished as its best ever." The result is notable in light of the economic downturn.

But such announcements have become a tradition for the Seattle-based online retailer. What was unusual this year was its peak sales date: Dec. 15. That's noticeably later than in years past.

Last year, for example, the company's busiest day was Dec. 10, according to its news release at the time. In 2006, it was Dec. 11, and in 2005, it was Dec. 12.

What's pushing back the peak date? Maybe people are growing more confident in the process of online shopping and shipping, delaying their purchases until the last possible moment. Maybe the uncertain economy made people a little more hesitant in their gift-giving. Or maybe it was quirk of the bad weather, with people scrambling to make last-minute online purchases after they realized they weren't going to make it to the malls.

[Update: Actually, an astute reader points out in the comments below that it's a quirk of the calendar. The peak day is typically the Monday about two weeks before Christmas, and this year that day happened to come later in the month. But given the limited number of days between that Monday and Christmas this year, it's notable that people still felt comfortable pushing the deadline.]

At any rate, Amazon's data for the busiest day is a rough measure, because we don't know the total dollar volume or the precise mix of items that make up the number. But this year, the company says 6.3 million items ordered on the peak day, up from 5.4 million items on the peak day a year ago. Comprehensive sales data won't come until Amazon's fourth-quarter earnings release.