At the time appointed for the hearing, or at the time to which the hearing may have been postponed, the court, unless the parties appear, must require proof that the notice has been given, which being made, the court must hear testimony in proof of the will. If such notice is not proved to have been given, or if from any other cause it is necessary, the hearing may be postponed to a day certain. The appearance in court of parties interested is a waiver of notice.