Anyone can register a domain at any time and, unfortunately, once it’s taken, it’s taken. There is also no way of telling when it might become available again since the current registrant may choose to renew it at any time before its expiry date.
However, if the domain is registered but not being used (i.e. ‘parked’), you can try offering to buy it from the current registrant by contacting them via our WHOIS Lookup tool.
Keep in mind that a domain name that is unavailable for one domain extension may still be available on any number of other domain extensions.