Complex sentence: Estonia completed their ratification on 3 October 2012, six days after the treaty entered into force.The inaugural meeting of the ESM didn't take place until 8 October, thus after entry into force for Estonia.
Split into simpler sentences: Estonia completed their ratification on 3 October 2012, six days after the treaty entered into force. However, the inaugural meeting of the ESM didn't occur until 8 October, after the treaty's entry into force for Estonia.