Usage patterns
〈事〉が盛りを過ぎるfor something to have passed its peak
The cherry blossoms have already passed their peak.
〈事〉が盛りを迎えるfor something to reach its peak
The autumn leaves are now at their peak.
今を盛りにat the very height of the moment
The cicadas are chirping at the height of the season.
Expressions
夏の盛りheight of summer; hottest part of summer 今を盛りとin full bloom right now; at its peak