Random.rand(日付の期間..日付の期間)
で取得できる。
irb> require 'date' start_day = Date.new(2020, 6, 15) last_day = Date.new(2020, 9, 15) irb(main):003:0> Random.rand(start_day..last_day) => Sat, 05 Sep 2020 irb(main):004:0> Random.rand(start_day..last_day) => Fri, 03 Jul 2020 irb(main):005:0> Random.rand(start_day..last_day) => Fri, 24 Jul 2020