Works even better in Ruby, as the code as given is valid, you just need to monkey patch length:

#!/usr/bin/env ruby

module DayLength
  def length
    if ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"].include? self
      "24 hours"
    else
      super
    end
  end
end

class String
  prepend DayLength
end

day = "Monday"

x = day.length

print(x)