Summer solstice at the Globe Monument in Mageroya, Norway | Shutterstock.com

Each day for six months after the winter solstice, the Sun rises a bit higher in the sky. It reaches the maximum height around June 21 in the northern hemisphere and December 21 in the southern hemisphere. This is the summer solstice, the longest day. Evidence of rituals and festivals at the times of the solstices goes back thousands of years.

What causes the solstices?
The Earth turns on its axis, but the axis isn't straight up and down. It's tilted 23.5 degrees. As Earth orbits the Sun, the direction of the tilt doesn't change. So at the time of the June solstice the north pole's full tilt is towards the Sun, and the south pole's is away from the Sun. The northern hemisphere has its longest day and the southern hemisphere its shortest one. This is reversed in December when Earth is at the opposite side of its orbit.

In astronomy, a solstice is the actual time at which the Sun reaches its greatest distance from the celestial equator. The celestial equator is a projection of Earth's equator into the sky, part of a celestial coordinate system that mirrors our earthly one. The apparent path of the Sun through the sky is called the ecliptic and it's at an angle because of the Earth's tilt.

Midsummer's Day
Astronomically, the summer solstice is the start of summer. Yet an old name for the day is Midsummer's Day, reflecting a tradition in which a solstice was in the middle of a season.

People celebrated the winter solstice to show their gratitude to the Sun and to petition his return. The bonfires of the summer solstice were not only a prayer to the Sun, but a bit of magic to strengthen his power to ensure a good harvest. The Midsummer festivals were about fertility of the land and the people.

There is evidence of Midsummer rituals going back to ancient times and it's still a popular holiday in many countries. Interestingly, the English-speaking world doesn't participate to any great extent.

St. John's Eve
Christmas contains many elements from the pagan winter solstice holidays that persisted when Christianity was adopted. The same thing has happened with St. John's Day on June 24. It celebrates the nativity of St. John the Baptist, but although most of the festivals took the name, they remained quite pagan.

It's common to celebrate – or start celebrating – on June 23, St. John's Eve. Why? It's a remnant of the time when people reckoned a day as ending at sunset. This is the origin of Christmas Eve and New Year's Eve.

St. John's Eve celebrations
Here are a few examples of the many contemporary celebrations.

Midsommarafton – Sweden
Except for Sweden, in Scandinavia and the Baltic countries the festival is named for St. John. Sweden has kept the old name of Midsummer's Eve. It used to occur on June 23-4, but now in both Sweden and neighboring Finland, it's a legal holiday whose date varies. It's celebrated on the Friday and Saturday nearest to St. John's Day. For the Swedes, at home and elsewhere, in addition to bonfires and other customs, dancing around a maypole is an unusual central festive feature.

Jāņi - Latvia
Jāņi, derived from the Latvian for John, is a major holiday, It involves special foods, singing, bonfires and jumping over the fire. People wear wreaths – flowers for women and oak leaves for men. They decorate their houses with greenery and in the countryside, their animals. Some decorate their cars too! Despite being named for the saint, there is more evidence of the old concerns of what the future holds in the way of love and economic security.

Sankthansaften – Denmark
St John's Eve used to involve pilgrimages to water wells with healing powers. Village healers took advantage of the magical power of the solstice to collect medicinal herbs which would be unusually potent. But magic could work against people too, so bonfires kept malign spirits away. In the Viking era bonfires were made on a beach, but other sites can be used now if you're lacking a handy beach.

La Noche de San Juan – Spain
Spain is known for its festivals, and certainly St. John is not omitted. There are bonfires, beach parties, fireworks, food, singing and dancing. As elsewhere, water and fire are prominent. There are old rituals of spiritual purification such as jumping over the bonfires or swimming in the sea at midnight.

Festa Junina – Brazil
São João (St. John's Day) was part of Brazil's big June Festival. This is surprising, as June is winter in the southern hemisphere. However although some parts of the country are wintry then, most of Brazil is in the tropical zone where the temperature doesn't fluctuate much throughout the year. Portuguese settlers brought São João with them and the celebration includes dancing, feasting, colorful clothing and of course, bonfires and fireworks. It's second only to Carnival in its importance to the Brazilians.

Sommersonnenwende - Austria
For my last example, I've chosen the one I would most like to attend. Imagine being on a ship in a parade of ships sailing down the Danube through the Wachau Valley on Sommersonnenwende (Summer Solstice). The vineyards are softly illuminated, candles float in the river, there are bonfires on the hills and the river banks, and fireworks pattern the skies.

Where would you like to celebrate the longest day?