One of Tokyo’s most famous gardens, Rikugien holds a light-up event twice every year—once to show the cherry blossoms and again in autumn to display the colours of the beautiful Japanese maple trees. The event starts at sundown and ends at 9pm, taking a fixed route around the garden, but note that there are only a few cherry trees in Rikugien—the most famous being the weeping cherry blossom tree (シダレザクラ) near the entrance. See here for a run-down of places to see the cherry blossoms in Tokyo.
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.