Forex trading is available 24h/day from Sunday night until Friday night. Keep in mind, though, that liquidity and volatility of the various pairs is influenced by the time of day! Certain hours are better for trading than others. Mornings are more volatile, so there's more money to be made then.
Here are the three main trading sessions. (in GMT (Greenwich mean time: London/Ireland) - winter time - ALL TIMES ON THIS PAGE ARE IN GMT, adjust accordingly for your time zone!)
Asia: midnight - 9:00
Europe: 07:00 - 17:00
US: 13:00 - 22:00
Overlap Asia / Europe 07:00 - 10:00
Overlap Europe / US 13:00 - 17:00
Trading hours in wintertime (GMT +0), in summer adjust to GMT +1
Volumes processed on the Forex market increase significantly during overlaps. The overwhelming majority of transactions pass through London, which is the leading financial spot, handling 34% of all currency trades.
From 7:00 until 10:00, the European financial markets are awakening while Asian markets are closing.
Between 13:00 and 16:00, volume on the following pairs is at its maximum: EUR/USD, GBP/USD, USD/CHF and EUR/CHF.
Between 2:00 and 7:00 in the morning, activity is high during the overlap of Asia and Australia's markets.
Between 21:00 and 23:00, there is no overlapping between markets. Opportunities aren't as exciting as volume is low during this time.