Said to be the place to go for all things vintage, this is a fairly large market just behind the Tokyo Metropolitan Government Buildings, a famous landmark and free viewpoint. It’s held 1-3 times a month, most months of the year.
Market times vary, with 10-3pm being a safe bet for scheduling. On some days, it starts at 9am and ends between 3:30 and 4pm—for early risers who don’t want to miss out on top treasures, you can always show up early and go for a coffee if nothing is happening yet.
Make sure to check the events page of the park beforehand in case of cancellations due to, e.g. rain.