The Rikugien Gardens usually closes at 16:00 as most Japanese Gardens do but they have a special period twice a year in which they close at 21:00 and light up key portions of the park for you to enjoy at nighttime.
In late March to early April, the park is open until 21:00 so that you can admire their weeping cherry tree. The color of the blossoms are very unique, as the blossoms are a shade deeper than the Somei Yoshino cherry blossoms you will see around the station. Unfortunately, I’m not a color wizard but the closest might be thulian pink.
I recommend going to the park around dusk so you can enjoy both the daytime and nighttime view.