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height (e.g. of summer); peak (e.g. of cherry blossom season); (in) season; full bloom; full swing (of a party, etc.)
prime (of one's life); (one's) best days; bloom
(being in) heat; rut
公園の桜が盛りを迎えている。
The cherry blossoms in the park are at their peak.
今は仕事の盛りで毎日忙しい。
It's the peak of work now, so I'm busy.
夏の盛りに家族で旅行に行った。
I went on a trip at the height of summer.