Ruby で月末、月初を取得

require "date"

#月末
p last_day = Date.new(2020, 8, -1)

#月初
p first_day = Date.new(2020, 8)

$ ruby dates.rb 
#<Date: 2020-08-31 ((2459093j,0s,0n),+0s,2299161j)>
#<Date: 2020-08-01 ((2459063j,0s,0n),+0s,2299161j)>

日付が入った配列を作りたい場合。

dates = []

(first_day..last_day).each do |date|
  dates << date
end

puts dates

結果

2020-08-01
2020-08-02
2020-08-03
~