Movable feasts

If you really want to upset someone, mess up their holiday dates.

Despite that being well-known, ten years ago our local council decided to do just that, allegedly in transition to a six term school year. In fact, whatever they pretended it to be, it boiled down to fixing an Easter-like holiday to the solar calendar, instead of to the religious festival. So parents who were obliged to take their holiday in relation to national public holidays, found themselves back at work when their children were at home from school. How absurd.

Easter, as with many other religious holidays of a wide range of different faiths, is linked to the lunar calendar, having been defined in 325 by the Council of Nicaea as “the first Sunday after the first full moon occurring on or after the spring equinox”. Although that sounds simple, the Easter definition industry got hold of it and constructed much more elaborate conditionals and case law. Standard Gregorian Easter is most likely to fall on 19 April, and least likely on 22 March.

And Sod’s Law will make sure that it seldom if ever coincides with our New Revised Standard School Holidays.

If forecasting Easter sounds like a good job for a computer, then so are the much greater subtleties of the calendars of other faiths. The Hebrew calendar is fundamentally lunar, but in order to bring Passover to the spring, it must also harmonise with the solar calendar. The Islamic calendar is thankfully a straightforward lunar calendar, but as this gives it less than 355 days in an average year, it, and the dates of Ramadan and festivals, migrates out of synchronisation with the secular Western (Gregorian) calendar.

These and other more arcane calendars are laid out in explicit detail in the third edition of Dershowitz and Reingold’s superb Calendrical Calculations (ISBN 978 0 521 70238 6), described on its companion website, and further supported here.

Whilst Dershowitz & Reingold offer their calendrical calculations in the great but relatively obscure programming language Common Lisp, with the second ‘millenium’ edition of their book they also provided a Mathematica Notebook supporting almost every date calculation known to man.

So it is that I can proudly inform you that on Monday 24 August 2015, it will also be Do-shanbeh 2 Shahrivar 1394 AP in the Persian calendar, and Erkoushabathi 2 Hori 1465 in the Armenian.

Mathematica, now running at version 10, is a remarkable mathematical environment that has grown from the vision and labours of one man, Stephen Wolfram. A Londoner and child prodigy, Wolfram shot through Eton, Oxford, and Caltech, to receive his PhD at the age of 20.

Mathematica’s origins were in a computer algebra system that Wolfram started to develop in 1979, and it was launched as a commercial product in 1988. Although snubbed by Claris who declined to distribute it, ever since then, through thick and thin, Mathematica has enjoyed excellent support on the Mac. It was also an early convert to and bundled with Steve Jobs’ NeXT systems. Given its pervasiveness in academia and industry, Wolfram Research and Mathematica’s staunch support for the Mac has been vital in ensuring the Mac’s survival.

There seems little that the products of this partnership of opposites, Steve Jobs and Stephen Wolfram, cannot tackle. Except of course for the crass stupidity of councils that go messing up school holidays.

Updated from the original, which was first published in MacUser volume 21 issue 11, 2005.