"Io Saturnalia!" Two thousand years ago this was the seasonal greeting which would have chimed out across most of Europe, not "Merry Christmas". The Roman mid-winter festival of misrule has heavily influenced many Christmas traditions - including the time of year we celebrate.

At no point is a date for Jesus's birth given in the Bible, but references to the lambing season have led some theologians to conclude that he was born in spring. Why then do we celebrate his birth in the middle of winter?

"Christmas in December is a Western, Roman idea whereas in the Eastern Church it falls later, around the feast of the Epiphany in early January," explains Dr Matthew Nicholls, senior lecturer in Classics at the University of Reading.

For seven days from the 17 December it was party season in Roman times. Homes were decorated, parties held and slaves became masters - at least for one banquet. It was the start of a lengthy mid-winter period of merry-making and the season of goodwill - Saturnalia.

Saturnalia originated as a farmers' festival and commemorated the dedication of the temple of Saturn, the Roman god of agriculture and the harvest.

During this festival, there was a reversal of traditional roles, with slaves wearing fine garments and sitting at the head of the table. Families gave each other gifts, and homes were decorated with wreathes and greenery. Gambling was allowed and the festival is described as a joyful period.

Over-eating, drinking, singing and gift-giving are all things that we associate with Christmas - another, more modern, season of goodwill.

"The Christian Church appropriated quite a few Pagan festivals and Pagan activities," according to Sam Moorhead, national finds adviser for Iron Age and Roman coins at the British Museum.

So many of our Christian traditions can be traced to Roman mid-winter festivals that a time-travelling centurion would feel quite at home sitting around the table for the Christmas banquet or joining in office party revelries.

"People would go round the streets and there was merry-making and singing songs, which some people associate with modern carolling," adds Mr Moorhead.

"You were also not allowed to give lectures at the time, unless they were witty or funny - which could be seen as the origin of cracker jokes."

Originally a one-day feast at the end of autumn, Saturnalia gradually moved to later and later dates, with longer celebrations, throughout the Roman period.

By the time of Christian conversion it was running into and incorporating a number of festivals. These included the Opalia - the festival day for Saturn's consort Ops - on the 19 December and the Sigillaria- the day of present-giving - on the 23 December. The 25 December was dies natalis solis invicti - the birthday of the 'invincible' Roman sun-god Sol.

Cancelling Saturnalia was unthinkable, so Christian Rome converted it to a Christian holy day instead.

"If Christianity moves Christmas into December, at the Saturnalia and the birthday of Sol, you can then fade out these other festivals and incorporate elements into the Christian festival. You can attempt to move on as if nothing has happened." explains Mr Moorhead.

To ban or not to ban?

Pagan ritual and Christianity coexisted for many decades after the conversion of Constantine the Great. This period was not always harmonious, especially in the fourth century, with tense and bloody episodes between the new Christian elite and those who still worshipped the old gods.

Festivals like the Lupercalia - held in February - where men ran through the streets of Rome naked, whipping women with strips of goat-hide, were still marked by Pagans and Christians alike.

According to Dr Nicholls, it may have seemed expedient for the new Christian ruling classes to allow Pagan traditions to be merged with the new state religion.

"Pagan temples and shrines were a frequent focus of religious conflict, and bans or interdicts provoked riots and civil unrest.

"In that climate a complete ban on Pagan worship would be provocative, and allowing some festival practices to continue under a new religious regime would be a way of softening the transition."

But there have been more modern attempts to ban aspects of Christmas because of their links with Paganism.

Carolling, or rather the dancing and drinking that often accompany it, has yo-yoed in and out of favour with Churches because of its association with the more debauched side of pagan festivals like Saturnalia.

The Puritan-led Parliament of Oliver Cromwell banned Christmas in church - except for a plain service of prayer. Dancing, feasting, gift-giving and carol concerts were doubly damned as 'Papal and Pagan'. The edict was incredibly unpopular and often openly flouted until it was overturned during the Restoration.

'Too embedded'

"The social need for a festival of some sort around that date [mid-winter] would have made Saturnalia or Christmas a powerful social institution, as it still is. Too popular and embedded to be easily done away with." explains Dr Nicholls.

As well as Roman traditions, rites and rituals from the Vikings and Anglo-Saxons have survived into our modern celebration. The tradition of kissing under the mistletoe is sometimes attributed to the Viking goddess of love and marriage - Frigg - whose legend is associated with the plant. The origin of the traditional Christmas tree may also linked to Pagan tree-worship.

While such traditions have been amalgamated into Christmas festivities the influence of the Romans, especially in terms of the festival calendar, is clear.