Cryptocurrency Market Never opens, never closes. It is functional and operational 24/7, 365 days.
Its the coin exchanges which take downtime due to infrastructural updates/forks which will be notified prior. at the same if you have accounts in multiple exchanges you can happily trade in the mean time.
Unlike other centralized markets or any trading stock markets, cryptocurrency market can't be 'closed'. It has no holidays or breaks. Some centralized exchanges may shut temporarily to update the servers, etc but these moments are really rare. Online crypto exchanges are way more dynamic than stocks-and-bonds ones.
Crypto market is open 24/7 across a growing number of exchanges. We can only calculate the 24-hour price change by comparing the current market price to the price a given cryptocurrency was trading the day before at exactly the same time.
See also: How to read crypto markets?
See also: Does crypto market cap matter?