When does Christmas start?

We've only just had Hallowe'en, bonfire night is not officially until tomorrow and we have 51 days until Christmas.
The John Lewis Christmas ad from 2014The John Lewis Christmas ad from 2014
The John Lewis Christmas ad from 2014

However, rumours suggest the much anticipated John Lewis advert could air today and House of Fraser has just released its Christmas offering. So does this mean it’s officially Christmas? When does the season officially start for you?