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Three billion pages, with over 100 factors in the algo, makes for a hell of a lot of calculations. Then there is the testing for how tweaks in both the indexing algo and the search algo turn out. This is also happening throughout the month. If any of those tests show a problem, they have to back out the data to a previous step.
Then when everything looks like it is at least as good as the previous month in most areas, then the real processing starts. There ain't no big red button that starts the dance that we see. Look at the times that the dance has started. If it was not automated, then it would always happen before noon pacific time so that they could monitor it. The dance is well past the point of no return. In fact, I would be willing to bet that the point of no return was before Christmas.
The 5 days of the dance are probably just the recalculation of PageRank.