Reports that I have seen all say that the (outgoing) Emperor will deliver a speech and receive "representatives of the people" in audience. I have not seen anything about foreign guests. I get the impression that this occasion is being treated as being between the Emperor and his people. The enthronement in October would be the more likely time for foreign dignitaries.
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On the last day of this month Emperor Akihito will abdicate and his son Naruhito will be enthroned on the following day.
Will there be foreign royals attending these ceremonies or will they just be invited for the coronation in october?