require "date"

days = (Date.parse("2020/01/01")..Date.parse("2020/01/04")).to_a
pp days
# => [#<Date: 2020-01-01 ((2458850j,0s,0n),+0s,2299161j)>,
#     #<Date: 2020-01-02 ((2458851j,0s,0n),+0s,2299161j)>,
#     #<Date: 2020-01-03 ((2458852j,0s,0n),+0s,2299161j)>,
#     #<Date: 2020-01-04 ((2458853j,0s,0n),+0s,2299161j)>]

# 2015/01/01 ~ 2015/12/31 間の日付を全て列挙して配列にする
days = Date.parse("2015/01/01")..Date.parse("2015/12/31")

# #count メソッドを使う事で『条件にマッチする要素』の件数をカウントできる
puts days.count {
  _1.month == _1.day / 10 + _1.day % 10
}