In the forex market, currencies from all over the world can be traded at all times of the day. The forex market is very liquid, and the increased availability of advanced technology and information processing has only increased the number of participants.
Although markets in many foreign countries are closed at the time when North American markets are open, trading on foreign currencies can still take place. While the majority of trading on a particular currency occurs when its main market is open, many other banks around the world hold foreign currencies, enabling them to continue to be traded at times when the main market is closed. For example, the North American markets are open when the Japanese markets are closed, but North American traders are still able to buy and sell Japanese yen through their brokerages and banks. However, the market for Japanese yen is more liquid at times when the Japanese market is open.
Certain currencies have very low rates of demand for exchange purposes. As a result, these currencies can be difficult to trade and can usually only be traded in specific banks. Because currency trading does not take place on a regulated exchange, there is no assurance that there will be someone who will match the specifications of your trade. However, the major currencies of the world, such as the American dollar, the euro and the Japanese yen, are the most widely available.
For further reading, see Getting Started In Forex, A Primer On The Forex Market, Wading Into The Currency Marketand Common Questions About Currency Trading.