The overwhelming reason most people visit Mito is the plum blossom season, which runs from mid-February through late March depending on the year's temperatures. The Mito Plum Festival is held over roughly a month in this window, and the best weeks — when multiple varieties are blooming simultaneously — are typically the last two weeks of February and the first week of March. On clear weekends during peak blossom the garden is crowded, but the spectacle justifies it.
Cherry blossoms follow in early to mid-April. The combination of plum and cherry seasons makes late February through April the prime visiting period. Autumn foliage (late October to mid-November) is pleasant, and summer in Mito is hot and humid. Winter outside the festival season is cold and unremarkable. If you can only come once, aim for the plum festival.