October Sakura (Juugatsu Zakura) is Sakura which blooms twice a year: Spring and Winter. One third of flowers bloom between the end of October to the middle of January, and rest of them bloom in April just like other Sakura. The flower of October Sakura is "Yae, 8 layers" and it has 5 to 18 petals.
There is another Sakura which also blooms in the same period. It is Winter Sakura (Fuyuzakura). The difference between them is the flower. The flower of Winter Sakura is "Hitoe, single layer" and has 5 petals.
Because they bloom in winter, both of them are commonly called Winter Sakura.