A candle in the window is a symbol of warmth and security. Traditionally, it marked the love of a family waiting for the return of a family member who was away from home.
In Colonial America a candle in the window was used to honor dignitaries, announce births and just plain celebrate, which might explain why we see more of them during this celebratory time of the year.
We think this sense of warmth and security conveyed by such a simple gesture is because a lit candle in the window tells you a few things: There is someone on the other side tending to it. The candle is in a window for others to see - it is reaching out from the home and communicating something to those outside of the home.
Do you have a candle-in-the-window tradition in your home? Electric or wax (seems the majority we see are electric.)?