Fixing parents' evenings? Few things are more frustrating than finding out too late that a fair proportion of parents couldn't possibly make it because the evening clashes with a festival in their religion.
Planning assemblies? No matter what time of year, it is handy to be able to say with confidence that "around this time of year, the festival of . . . falls. We can learn so much from it."
No school with large numbers of pupils from diverse religious communities would want to miss a key event. And where there are only a few, it is critical that the dates marking out their time should be understood by teachers - who may otherwise be puzzled as to why that Jewish child is off "ill" so often in September.
The Christians in the school must be taken seriously as well, of course. Failing to wish your Greek Orthodox pupils happy Christmas after the Christmas holidays is to miss an important point about Christian diversity.Key saints' days, now forgotten by many, give opportunities to address, for example, the nature, value, history (and challenges) of the distinctive identities in the four nations within the British Isles.
A convenient route to understanding all this is the Shap Calendar of Religious Festivals, published each summer by the Shap Working Party on World Religions in Education. This little booklet, together with its colourful wall chart, gives the dates of festivals from about a dozen traditions, and includes brief notes on each festival. The calendar has cultivated an enviable reputation over the past 20 years as the leading provider of dates, not only for schools, but for law courts, social services and diary producers.
It is hardly surprising that so many people rely on the Shap booklet. The various calendars - Chinese, Christian Orthodox, Bah' (19 months of 19 days each, with a spare five at the end), Jewish, Hindu, Muslim, Rastafarian, Zoroastrian, and the rest - appear to move around with little rhyme or reason to the untutored eye.
We are mostly familiar with the two calendrical cycles in Western Christiani ty - Christmas and Easter. They work on separate systems. And although we're fairly confident about predicting Christmas, we'd need a diary to find out when Easter takes place.
Most Eastern traditions delay fixing their dates until the spring astronomical features have been sighted, which is why Shap doesn't publish until the summer. Most chancy is Shap's provision of dates over 18 months. When I edited the calendar, I frequently had to explain to Hindu temples why I couldn't tell them the date of Divali two years hence. And when I once phoned a Chinese cultural centre to check my date for Chinese New Year, they couldn't confirm it because they hadn't received the Shap calendar.
Some traditions cut it even finer. Muslims might arrive at the mosque for the fast-breaking festival of Id-ul-Fitr only to be told the new moon indicating the end of Ramadan hasn't after all been sighted and that they should return the next day. For such reasons, even the dates in the Shap calendar sometimes need to be checked near the time, but it gives full advice on when this might be necessary.
Diary producers, with their long lead times for publication, are among those most frustrated with the "inefficiency" of various traditions in not fixing their dates. I have often given publishers a best guess only to find a leap month has been introduced when it was least expected and my calculations for one religion or another have been a month out.
Calendars are important. Often they provide the clearest insight into what matters to a tradition, culture or religion. Recent debate about how - even when - to mark Armistice Day is just one local example. Discovering that many Hindu festivals take place on different days for different communities, or focus on different aspects of the celebration, drives home the vast diversity of Hinduism and its loose arrangement. Buddhism, on the other hand, has distinctly national identities, showing the local and personal nature of its commitment. Vietnamese Buddhists, for example, observe festivals that Tibetan or Western Buddhists might not take seriously, or observe on a different date.
Jews, Muslims, Sikhs, Rastafarians, Jains, Shintos and Bah's are all pretty well disciplined calendrically, reflecting their sense of particularity and identity, while what their festivals celebrate displays their aspirations for humanity as a whole.
But probably the most confusing of all calendars is that - or rather, those - of Christianity. Besides the Christianity of the West, there is an Eastern tradition, with its own way of calculating dates. Then there are those who still adhere to the Julian calendar, ignoring the extra few days Pope Gregory thrust upon us all in the 16th century. The drawing back of the Iron Curtain and the resurgence of Christianity in the East is something we should all understand, if for no other reason than that we shouldn't again be taken by surprise by conflicts such as that between the Serbs and the Croats.
The Shap calendar provides a quick and easy way of sensitising ourselves to the diversity of the world at large and our own little corner of it. Little wonder that each year thousands of teachers buy it and use it.
Clive Lawton is a freelance educational and management consultant. He was editor and compiler of the Shap calendar from 1982-95
The Shap calendar can be obtained from Shap Working Party, co The National Society's RE centre, 36 Causton Street, London, SW1P 4AU