A Piercing line candle pattern is a 2 day pattern consisting of a red candle, then followed by a green candle. The second candle must close above the midpoint of the first candle. (This signifies that buyers overwhelmed sellers on this day.)
The pattern is preceded by a downward trend in price. (This may be only a short downtrend, but if the candles appear after an upward trend in price it is not an important)