each_with_object.rb

(1..31).each_with_object({}) { |day,hash| hash[day] = nil }
=> {1=>nil,
 2=>nil,
 3=>nil,
 4=>nil,
 5=>nil,
 6=>nil,
 7=>nil,
 8=>nil,
 9=>nil,
 10=>nil,
 11=>nil,
 12=>nil,
 13=>nil,
 14=>nil,
 15=>nil,
 16=>nil,
 17=>nil,
 18=>nil,
 19=>nil,
 20=>nil,
 21=>nil,
 22=>nil,
 23=>nil,
 24=>nil,
 25=>nil,
 26=>nil,
 27=>nil,
 28=>nil,
 29=>nil,
 30=>nil,
 31=>nil}