The Evening Star is a three day pattern i.e it has three candlesticks. 

The first day is a long bullish candlestick after a preceding uptrend.  Second day is a little  bullish or bearish candlestick. It gaps up i.e opens at a higher price than the first day’s closing price. 
Third day is a large bearish candlestick and it closes into the first day’s bullish candlestick real body, ideally well down into it.

Psychology of the Evening Star pattern is that the market is on an uptrend  and on the first day of the pattern is a large bullish candlestick which reinforces the existing uptrend.
Next on the second day the candlestick opens higher with a gap up showing bulls in control but bears take control and manage to control the price push. This results in a small real body of the candlestick.
On the third day, bears take control and push prices down creating a long bearish candlestick and change in market sentiment to downtrend.

If the second day candlestick is a Doji then the pattern is called Evening Doji star. Evening star and Evening Doji Star are top reversal patterns.

Evening Star Evening Star-1 Evening Star-2 Evening Star-3

Practice Evening Star – Beginner

Practice Evening Star – Intermediate