$ ruby --version
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin10.8.0


require 'date'

class Dia < Date
  def initialize
    #@mes = obj_mes
    #super(ano, mes, dia)
    puts 'init'
  end
end

Dia.new

--output:--
<nothing>

So that means Dia#initialize is not being called by Dia.new, which is a 
bit confounding.  Apparently, the date module does some complex aliasing 
which causes Class.new (which Dia inherits) not to call initialize(). 
Your subclassing of Date problem was discussed in 2001, by none other 
than Matz, and supposedly that behavior was going to be fixed:

https://groups.google.com/forum/?fromgroups=#!topic/comp.lang.ruby/sDiFAuBjuwY

It seems to have been unfixed in ruby 1.9.3.

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