The Member States 25 March 1957, Consolidated version of the Treaty on the Functioning of the European Union#PART SEVEN - GENERAL AND FINAL PROVISIONS#Article 358

This Treaty shall enter into force on the first day of the month following the deposit of the Instrument of ratification by the last signatory State to take this step. If, however, such deposit is made less than 15 days before the beginning of the following month, this Treaty shall not enter into force until the first day of the second month after the date of such deposit.