The answer to this question is somewhat complex. In order for the mourning ceremony to take place, the person must be declared dead and then a death certificate must be issued.
If the person concerned is declared to be missing and within 5 years of the end of the year the disappearance is not found and there are no reports, by the law the court can declare him / her dead. If the person concerned has not been declared missing, this period extends to 7 years.
If your relative dies in result of an accident where the deceased could not be found (for example, a plane crash), the court decides whether to declare the person dead based on suggestion from a person who has a legal interest in it.