Active trading sessions in each country’s’ financial centers around the world take place from Sunday 5:00PM EST to Friday 5:00PM EST. For the major financial centers, trading starts in Sydney, then moves to each financial center in this order: Tokyo, London (and Europe), New York. The daily session for daily charting purposes “ends” at 5:00PM EST (coincident with the New York “close”), but the market does not actually close. Here are the time intervals for each of the major financial centers expressed as EST.

Sydney session starts at 5:00 pm and ends around 2:00 am.
Tokyo session begins at 7:00 pm and ends around 4:00 am.
London opens at 3:00 am and ends around 12:00 am.
New York session opens at 8:00 am and ends around 5:00 pm.

To give you a visual representation of this, here’s a figure showing the same business hours for the various regions. In this figure you can see the overlap between the London (and Europe) session and the New York session, between 8 am and 11 am EST. The currency markets experience the highest volatility and volume during that overlap, which also coincides with the releases of important US economic releases.