Iranians are preparing to mark the longest night of the year with a festival they have celebrated for thousands of years.

The night of Yalda, also called Shab-e Chelleh, falls on December 21 in the western calendar and was originally associated with the rebirth of the sun god. These days it is purely a social occasion for relatives to gather and mark what is generally regarded as the beginning of winter and also the moment the days begin getting longer again.

Families get together at home to eat fresh fruit and dried nuts, and to divine their fortunes by dipping into the poetry of Hafez. Red-coloured pomegranates and watermelons have pride of place, and make an eye-catching common sight in storefronts in the days leading up to the festival.

Despite the official emphasis placed on Islamic dates in the national calendar, Yalda has deep roots as a specifically Iranian festival, and in recent years some cultural centres have held public celebrations. The government declared the date part of Iran's national heritage in 2008.

The long dark night of Yalda is a recurring theme in Iranian literature and song, used by poets and singers to symbolise loneliness and separation from the beloved, and anticipation of changes to come.

Saadi, one of the great classical poets, wrote, "There will be no morning until Yalda passes," and, "With all my pain, there is still hope of recovery; Like the eve of Yalda, there will finally be an end."