No daylight saving, one zone
Japan stays on Japan Standard Time, UTC+09:00, every day of the year. There is no seasonal change and no regional variation — the whole country, from Okinawa to Hokkaidō, runs on the same clock. For anyone scheduling with Japan this is a considerable simplification: the offset from your own zone changes only when your side changes.
The brief American-era experiment
Japan did observe daylight saving from 1948 to 1951, introduced during the Allied occupation. It was deeply unpopular — associated with the occupation itself, and blamed for lengthening the working day in a culture where leaving before your superior was difficult — and was repealed as soon as the occupation ended.
It keeps being proposed
The idea resurfaces periodically, most recently in the run-up to the 2020 Tokyo Olympics, when there was a serious proposal to shift clocks two hours to move marathon events out of the worst of the summer heat. It was abandoned, partly over the cost of updating computer systems across the economy for a temporary change. Japan continues to have no daylight saving.