The availability of a market usually depends on the trading hours of the underlying market.
EUR/USD for example, closes at 21:59 UTC+1 on Friday night and re-opens at 22:00 UTC+1 on Sunday night. You can learn about the trading hours of each market on the website or in our app in the 'Trading info' section, when you click on a particular instrument.
As a general rule, markets do close at the weekend, so it is important to consider the risks of keeping a position open through Friday night. This is because news that could impact the financial markets could arise over the weekend and therefore cause a gap in the market when it reopens.