Goryokaku park is a historic site in Hakodate, Hokkaido. It is a former bastion fort called Goryokaku. It was a seat of commissioner of Hakodate and his officials and Ezo territory, appointed by Tokugawa shogunate.
After collapse of Tokugawa shogunate, the governor hall was used as the seat of Republic of Ezo, a short-lived separatist existed just half a year in 1869. Ezo is a former name of Hokkaido.
Today, Goryokaku park is visited by Hakodate citizens and tourists from all over Japan and gets crowded during sakura cherry blossom season each year. Sakura season is usually between end of March and beginning of April, Hakodate is beginning of May. Therefore if you cannot visit Japan in April but want to see sakura, you would still have a chance to see it in May in Goryokaku park.