Complex sentence: The tradition that some modern Christians observe, of lighting a candle in each window (or in each room), is not the origin of the name "Candlemas", which instead refers to a blessing of candles.
Split into simpler sentences: The tradition that some modern Christians observe, of lighting a candle in each window (or in each room). It is not the origin of the name "Candlemas" which refers to a blessing of candles.