The forex market is available 24 hours a day. It is open from 5:00pm ET Sunday until 5:00pm ET Friday.

This allows brokers and traders worldwide to trade 24 hours a day, with the involvement of central banks all over the world. It is the time when forex traders are capable to buy, sell, exchange and speculate the currency values. International currency markets are composed of central banks, retail forex brokers, investors, banks, investment management firms, commercial companies, and hedge funds.

Timezone (Summer)
Sydney Open/Close
6:00 PM 3:00 AM
10:00 PM 7:00 AM
Tokyo Open/Close
7:00 PM 4:00 AM
11:00 PM 8:00 AM
London Open/Close
3:00 AM 12:00 PM
7:00 AM 4:00 PM
New York Open/Close
8:00 AM 5:00 PM
12:00 PM 9:00 PM

Here are the time periods when two trading sessions overlap:

New York and London: from 8am to 12am (EST)
Sydney and Tokyo: from 7pm to 2am
London and Tokyo markets overlap one hour, from 3am to 4am

The above time periods are considered to be the busiest ones, for the reason that there is more volume when two markets are open at the matching time. Through this period of time, more and more trading deals are been held, meaning more funds are being exchanged. Note that Sydney open shifts 2 hours.

Hypothetically the session open would only move by 1 hour when New York changes to standard time, however when New York moves 1 hour back, Sydney actually moves forward by 1 hour as the seasons are opposite in Australia.