Thanks for the various responses, which are much appreciated. I have fixed thousands of problems over the years, but the vast majority are not that exciting - problems resolved by asking 'Is your Caps Lock on?' tend not to lend themselves to interesting articles. Although in the interests of accuracy, it would only be fair to state that my wife did not find this adding of dates problem all that interesting! Who knows, I may get a few more interesting problems to write about in due course.
I did have a look at a 'simplified' view of Scottish Public Holidays, and was shocked to see how complex it is. I can't think how it works out in practice, where folk in each town may have different holidays from the town down the road. I would think that somebody who lives in that environment would be far better placed to write the required tweaks to the Public Holiday Calculation. The specific Irish requirements by contrast look quite straightforward - I would add a 'Scope' field, which would describe which area would get the holiday (which could be pretty complex in Scotland!).
And thanks to ChrisM for his contributions. I can say that my query looked really good compared to some of the clunky queries that they used, but there is always room for improvement. My experience to date with parallelism has been somewhat mixed, although I don't doubt that it will at some point be the grail that we all pursue.