Easter is the most important Christian Festival as it celebrates Jesus re-establishing our relationship with God that had been spoilt by our inability to keep to his rules but, especially in western Europe, most people now put greater emphasis on the non-religious or traditional parts of Christmas, especially in shops, many of which start their Christmas activities several weeks if not months before the Christian festival. We have therefore explained Christmas in two parts - the religious festival and the
non-religious traditions.
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The church's season of Christmas lasts 12 days and runs between
Advent and
Epiphany. Unlike the modern commercialism of Christmas that begins several week, if not months, earlier, the church's celebration doesn't begin until Christmas Day - December 25th - and lasts for the next 12 days
(hence the Carol 'The 12 Days of Christmas'). It celebrates the birth of Jesus Christ and is the second most important Festival after
Easter (which is the most important), although most people prefer Christmas as it's a season of merriment and partying in the otherwise dull period of midwinter
(at least in the northern hemisphere). It gets its name from Christ-Mass, the church's Mass or Service for Christ.
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No-one knows the date of Jesus' birthday - most cultures have many festivals and the Roman Empire was no different, they had a Festival in winter called 'Saturnalia' after one of their pagan gods, Saturn. It involved much merriment and was very popular, so the early Christians who lived at that time adopted that date as the date for celebrating Jesus' birthday to direct the Christians into worshipping Jesus rather than Saturn.
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The season of Advent immediately before Christmas used to be austere, a time of reflection and preparation before the exertions of Christmas itself. In more recent times, there has been commercial pressure in the shops to sell Christmas products and presents, with corresponding emphasis on joyful Christmas activities rather than the more austere Advent ones. So in the secular world, Christmas preparation and celebration now tends to start some months before December 25th and occupy the period previously taken up by Advent.