At midnight, the .yu extension won’t exist any longer. Since Yugoslavia fell apart, it wasn’t possible anymore to register a domain name under the .yu domain but already existing names were still supported. At midnight, this support will also stop.
Since beginning 2008, people who own a .yu domain name were offered the possibility to switch this domain name to a .rs (Serbia) or a .me (Montenegro) name. People who haven’t done this, we’ll be facing a problem after midnight since not a single .yu website will still be reachable.
It seems logic, that once a country doesn’t exist anymore, it isn’t possible to register a domain name under this country code extension. However, it doesn’t always have to be like that. It is still possible to register a domain name under the .su extension (Sovjet Union) although this union doesn’t exist anymore for quite a while.