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.
As you may notice, there is a period of time where 2 sessions are overlapping, since they are open at the same trading hours.
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.