[DelaneyM]

Along other reasons, with such a massive percentage of their income (and income variability) in Q4, it really helps their accountants to close out a tax year in the (relatively) predictable and stable Sept after three quarters of normalizing cashflows.

Sheltering taxes in a double-Irish requires pretty accurate projections.

Also often they just don't have all the data they need to close out a year until a few weeks after holidays, especially if people are taking a break, so it's just a lot easier on the humans involved.