Ruby なら、

require 'csv'
require 'date'

input = <<"EOT"
year,month,day
2021,5,31
2021,6,2
EOT

options = { :headers => true } # ヘッダー有り
csv_ary = CSV.parse( input, options )

tmp_from = csv_ary[ 0 ]
tmp_to = csv_ary[ 1 ]

from_day = Date.new( tmp_from[ 0 ].to_i, tmp_from[ 1 ].to_i, tmp_from[ 2 ].to_i )
to_day = Date.new( tmp_to[ 0 ].to_i, tmp_to[ 1 ].to_i, tmp_to[ 2 ].to_i )

Range.new( from_day, to_day ).each{ |day| puts day }

出力
2021-05-31
2021-06-01
2021-06-02