Trading on Forex is done during market hours, otherwise known as sessions. There are three trading sessions on the forex market: the US session, the European session, the Asian session, and the Pacific session.
As one trading session closes, another one opens. There are some volatile hours when two sessions overlap. Thus, EU traders come from lunch at the moment when US traders just start morning trades.
The European session is the most volatile one. However, the most aggressive trading occurs during the New York session. The Pacific trading session is the least volatile. The Asian session has little activity, but is no less efficient.
Knowing Forex market hours will help you trade effectively, building your own trading strategy and understanding the features of trades during each trading session.