Massachusetts General Laws Part IV. Crimes, Punishments and Proceedings in Criminal Cases (Ch. 263-280) Ch. 277, § 2B

The clerk of courts in Middlesex county shall, not less than seventy days before the first Monday of January and July, respectively, issue letters of venire to the jury commissioner for thirty-five prospective grand jurors from whom the court shall select twenty-three grand jurors who shall serve in said court for each sitting thereof for six months and until another grand jury has been impanelled in their stead.