When the violet vestments appear to mark the start of Advent, Christmas is definitely on its way.
On a secular level, the harbingers are always the hot weather and the crowds of schoolchildren on the streets as the academic year ends and the school holidays start.
The banners at my church have a graduated theme of Mary & Joseph approaching Bethlehem. Each week it shows them closer and closer, and then on the Sunday before Christmas it changes to a nativity scene.
Thanks for reminding me of that - it was always one of my favorite parts of Christmas!