Setagaya's Hanegi Park is home to over 600 plum blossom trees. These small, pink flowers bloom when the weather starts to warm up and Hanegi Park celebrates with a month-long festival.
The Setagaya Plum Festival (or Setagaya Ume Matsuri) celebrates the flowering season with events like mochi pounding, tea ceremonies and traditional music performances. Most of these events take place on weekends and holidays. You'll find the event schedule here (in Japanese only).
While you're there, stop by the on-site food stalls for plum jam or plum-flavoured treats like sweet mochi daifuku or baked manju with candied plums and white bean paste inside.