Never open and never close it remain open 24/7 and you can place a trade at any time in a week. In fact, there is no regulator on the back end and it depends on the exchanges and that remain open for all time and there is no close time. If you want to buy or sell just put an order there and your order will be filled as price come to your level.
The markets that we experience until now that open and close for a specific time the most common reason behind is a regulator that define when to open and when to close but when consider crypto than there is not regulator its only exchanges and liquidity providers which have control over it. Even online currency brokers provide access to this market on off days. Even this market remain open on Christmas.