The forex market opens on Mondays and remains open 24 hours till Friday. There is low liquidity on weekends; therefore, traders don’t trade on weekends. And there are 4 major forex sessions, starting with the Sydney session. The liquidity of the market varies as per sessions. 50% of the trades occur when the UK and the US sessions overlap because there are more pairs to trade and more trading participants.
I might not be very correct but I think that since international currency markets are made up of banks, investment management firms, commercial companies, hedge funds, investors as well as retail traders, all of which actively operate as soon as the week starts. As we all know that London and New York sessions are the busiest and everyone is excited to trade the fresh week as early as possible, maybe to avoid that rush the forex market opens at 5pm ET on Sunday.
If we talk about the international time zones, they start in the morning and close in the evening, just like regular offices do. But now that there are four different time zones across the globe, the market stays open for 24 hours 5 days a week, giving traders the opportunity to trade whenever they want.
The forex market is open for 24 hours, five days a week. The liquidity varies depending on the trading session. It is mostly seen that volatility is high when a session starts so it's better to avoid it. You should trade during the liquid hours to minimise the costs and maximise the other benefits like execution speed.