Each year around mid-May, Yamagata's Sagae Park is home to an azalea festival which celebrates the impressive number of them on the grounds. In total there are over 40,000 azalea bushes covering 11 different types - it's one of the largest azalea displays in the entire Tohoku region. For those visiting the area earlier in spring, you'll be pleased to know there are also plenty of cherry blossoms to enjoy before azalea season.
As is the case with all flower events, the dates for the festival can differ annually depending on blooming conditions. Based on previous years, plan for the festival to take place around the third weekend of May - but do check the official site for a definite confirmation before attending.