Holy days and calendars that disagree
The word HOLIDAY began as holy day, a day set aside for religion, when work stopped for church services and feasts. The meaning widened over the centuries to any day off, and later to a whole trip away.
Holiday dates behave oddly because the world runs on different calendars. CHRISTMAS sits on a fixed date in the solar calendar that most countries use. EASTER moves because it is tied to the moon as well as the sun. Ramadan and Eid travel right round the year because the Islamic calendar follows the moon alone and is about eleven days shorter than the solar year. HANUKKAH, DIWALI and Lunar NEWYEAR follow calendars that use the moon but add an extra month every few years to stay in step with the seasons, so they wander within the same few weeks.
Where firework colours come from
FIREWORKS were invented in China, as our ancient China page explains, but their colours came much later. Early fireworks glowed only orange and white. In the 1800s, chemists discovered that metal compounds burn with their own colours: strontium makes red, barium green, sodium yellow and copper blue. Blue is the hardest colour to make well, because the copper compounds that make it break down if the firework burns too hot.
The shapes are planned too. A firework shell is packed with small pellets called stars arranged around a bursting charge, and the pattern they are packed in decides whether the explosion makes a sphere, a ring or even a smiley face.
Lights in the darkest days
A surprising number of festivals around the darkest part of the year are about light. In Sweden, on 13 December, Saint Lucia's Day, girls in white robes walk in processions wearing crowns of candles and singing, carrying saffron buns. In Iran, families stay up through Yalda, the longest night of the year at the winter solstice, eating pomegranates and watermelon and reading aloud from the poems of Hafez. HANUKKAH is the festival of lights, CHRISTMAS brings candles and fairy lights, and DIWALI, a little earlier in the autumn, fills windows with lamps.
The day two writers did not die together
World Book Day is held by the United Nations on 23 April, chosen partly because two of the world's greatest writers, William Shakespeare and Miguel de Cervantes, both died on 23 April 1616. Except they did not die on the same day. Spain had already switched to the new Gregorian calendar, while England was still using the older Julian one, ten days behind. Cervantes died on 22 April by the Spanish calendar and was buried on the 23rd; Shakespeare died on 23 April by the English one, so the two deaths were really about eleven days apart. Britain and Ireland hold their own World Book Day on a Thursday in early March, to avoid the Easter school holidays.
A year of celebrations
The grid holds celebrations from every season: HALLOWEEN and THANKSGIVING in autumn, a VALENTINE card in February, a BIRTHDAY that comes round once a year for everyone, and CARNIVAL before Lent. They share the same ingredients the world over: a FEAST, a PARADE, a COSTUME, DECORATIONS, PRESENTS, a PARTY and a FESTIVAL, all ways to CELEBRATE. Each has its own page in this category, and our world religions page covers the faiths behind many of them.