The annual bon odori in the Roppongi Hills Arena. A yagura (wooden stage around which people dance) is set up in the middle of the arena and there will be a good selection of booths selling food and drink. Stalls are open from 3pm to 9pm with dancing at the weekend between 5pm and 8pm (no dancing on the Friday). The event is free to join.
Event details are taken from official sources whenever possible. It is always prudent to check the official event website before attending to make sure that there have not been any changes. If you know of an error in the calendar, then please let me know through the contact form.