Mass. Gen. Laws ch. 41 § 115
Every nomination paper of a candidate for office in said districts shall, before being filed, be submitted, on or before five o'clock in the afternoon of the Saturday preceding the day on which it must be filed, to the registrars of voters of the town where the district is situated, who shall forthwith certify thereon the number of signatures which are names of voters qualified to vote in the district. All nomination papers and certificates of nomination shall be filed with the clerk of the district within the time allowed by the general laws relating to filing nomination papers and certificates of nomination with the town clerk in town elections. In a district composed of parts of two or more towns the prudential committee, if any, otherwise the commissioners, of the district shall act as registrars of voters for the purpose of certifying the signatures on nomination papers.
Mass. Gen. Laws ch. 41, § 115