The 24 hour FX Market is due to the fact that markets are open 24 hours a day in any part of the world at a given time. From the open in New Zealand, through Asia till the American closing in the last, trading is done 24/5 from Sunday evening till Friday Night.
The three main exchanges and sessions that produce most of the worlds FX volume are London Stock Exchange, Tokyo Stock Exchange and New York Stock Exchange. There are certain periods of time when these 3 sessions overlap each other and are open at the same time. This is the time when the market presents the best investment opportunities and the markets are more volatile. This image will help traders check the timings(in GMT 0) when the overlap happens and the market presents the opportunites to invest.