It’s important to note that the Advent relates not only to the original nativity of Jesus, but to the Christian expectation of his second coming in glory at the end of time. Hence Advent hymns like Charles Wesley’s and John Cennick’s ‘Lo He comes with clouds descending’, which relate to that apocalyptic expectation rather than the baby in a manger at Bethlehem.
The Church of England’s Collect (ie, special prayer) for Advent Sunday sums up this dual meaning well:
‘Almighty God, give us grace that we may cast away the works of darkness, and put upon us the armour of light, now in the time of this mortal life, in which thy Son Jesus Christ came to visit us in great humility; that in the last day, when he shall come again in his glorious Majesty to judge both the quick [ie living] and the dead, we may rise to the life immortal, though him who liveth and reigneth with thee and the Holy Ghost, now and ever. Amen.’
Traditionally Advent runs from Advent Sunday, the fourth Sunday before Christmas, which is always around 1st December – hence the convenience of treating Advent as starting on 1 December – but actually it seldom does precisely! So this year, Advent Sunday was actually 30th November.
Advent Sunday is also the start of the church’s liturgical year. Traditionally in the church Advent was a period of fasting leading up to Christmas (in the same way as Lent is leading up to Easter) but my impression is that it was never so widely observed. However the contemporary tendency to use Advent to anticipate Christmas rather than just prepare for it is at variance with historic Christian tradition.
Other present-day church/Christian traditions are to have Advent crowns (with five candles – one for each Sunday, and one for Christmas Day) and/or Advent candles (marked out with a section to be burnt each day of Advent).