And they won't know when it's going to happen until a few hours beforehand. If I'm not mistaken, the Japanese think that's less cruel than scheduling it for a known time.
Support for capital punishment is somewhere between 70% and 80% in Japan, but they do have their lefty lunatic fringe as well.
The announcement is usually a day or so after the fact pretty much simultaneously to when the next of kin are contacted to come down to the hoosegow and collect their personal effects and (if so arranged) their cremated remains.