At midnight tonight, the Tennō will be deposed.
Most western reports talk about his “abdication”, but legally, that is not what is happening. Legally, the Diet passed a law to depose the Tennō, and the Tennō signed it (not that he had any choice).
It is happening this way because, under the Constitution, the Tennō has no legal power at all, and may not influence any political or legislative events. The status of the Tennō is defined in the Constitution, and is thus one of the things that the Tennō himself may not legally influence in any way. He certainly may not decide to stop being Tennō — to abdicate. Nor may the heir decide not to become Tennō. (Well… Japanese law does recognise gender transitions, under strict conditions, and the Tennō must, legally, be male, so the heir could transition to being female to dodge it. I suspect that another solution would be found, however.)
In fact, the Cabinet legal office, in their interpretation of the Constitution, have said quite explicitly that the Imperial family do not have all the rights afforded to other citizens of Japan, and, for the most part, to foreign residents. In particular, I believe, they do not have the right to freely choose their occupation, nor, technically, the right to freely marry. There may also be legal restrictions on their freedom of speech and political participation; the current Imperial family are very careful not to intervene politically, but it may, in fact, be illegal for them to do so.
This is why the Tennō had to tread very carefully. In August 2016, he gave a superbly constructed speech in which he said absolutely nothing about ceasing to be Tennō. He merely observed that he was getting old (he was 82 at that point), and that he was concerned that he would not be physically or mentally able to perform his role as a symbol of the Japanese people for much longer. He also noted that performing the accession rites for a new Tennō at the same time as the funeral rites for the previous one put a lot of pressure on both the Imperial household, and the country.
Everyone got the point, and agreed that he should be allowed to retire. (Well, apart from some parts of the Shinto establishment, who appear to think that you should respect the will of the Tennō except when you disagree with him.) This led to discussions in and around the Diet, as people tried to work out a way to do this legally. Formally, none of this had anything to do with the Tennō’s speech, because he must not influence the political process. It was all a complete coincidence that people started looking into the problem at that point. Honest.
The final decision was to pass a special addition to the Imperial Household Law stating that the current Tennō would be deposed on a day fixed, within a certain range, by Cabinet order. The law passed unanimously, with merely formal discussion in the Diet.
So, in reality, the Tennō is abdicating, but the legal fiction is that he is being deposed. History is full of “abdications” that were not really voluntary, but this may be the first example of the reverse.