Forex market is open 24 hours a day. It provides a great opportunity for traders to trade at any time of the day or night. The best time to trade is when the market is the most active and therefore has the biggest volume of trades. Actively traded markets will create a good chance to catch a good trading opportunity and make profits.
When liquidity is restored to the forex market after the weekend passes, the Asian markets are naturally the first to see action. Unofficially, activity from this part of the world is represented by the Tokyo capital markets. However, there are many other countries with considerable pull that are present during this period including China, Australia, New Zealand and Russia, among others. Considering how scattered these markets are, it makes sense that the beginning and end of the Asian session are stretched beyond the standard Tokyo hours.
Later in the trading day, just before the Asian trading hours come to a close, the European session takes over in keeping the currency market active. This forex time zone is very dense and includes a number of major financial markets that could stand in as the symbolic capital. However, London ultimately takes the honors in defining the parameters for the European session. Once again, this trading period is expanded due to other capital markets' presence (including Germany and France) before the official open in the U.K.; while the end of the session is pushed back as volatility holds until after the close.
By the time the North American session comes online, the Asian markets have already been closed for a number of hours, but the day is only halfway through for European traders. The Western session is dominated by activity in the U.S., with a few contributions from Canada, Mexico and a number of countries in South America. As such, it comes as little surprise that activity in New York City marks the high in volatility and participation for the session.