Mass. Gen. Laws ch. 53 § 38
No voter enrolled under this section or section thirty-seven shall be allowed to receive the ballot of any political party except that in which he is so enrolled; provided, however, that, except as otherwise provided in said section thirty-seven, a voter may, except within a period beginning at eight o'clock in the evening of the twentieth day prior to any primary and ending with the day of such primary, establish, change or cancel his enrollment by forwarding to the board of registrars of voters a certificate signed by such voter under the pains and penalties of perjury, requesting to have his enrollment established with a party or political designation, changed to another party or political designation, or cancelled, or by appearing in person before a member of said board and requesting, in writing, that such enrollment be so established, changed or cancelled. Except as otherwise provided in section twelve of chapter four, sections one and two of chapter fifty-two, and sections forty A and forty-eight of this chapter, such enrollment, change or cancellation shall take effect upon the receipt by said board of such certificate or appearance, as the case may be; provided, however, that no such enrollment, change or cancellation shall take effect for a primary during the twenty days prior to such primary; provided further, that a voter registered in a political designation that is not a political party as defined in section 1 of chapter 50 shall be considered unenrolled for the purpose of the primary and shall be eligible to receive a ballot of a political party of the voter's choosing. No voter enrolled as a member of one political party or political designation shall be allowed to receive the ballot of any other political party, upon a claim by him of erroneous enrolment, except upon a certificate of such error from the registrars, which shall be presented to the presiding officer of the primary and shall be attached to, and considered a part of the voting list and returned and preserved therewith; but the political party or political designation enrolment of a voter shall not preclude him from receiving at a city or town primary the ballot of any municipal party, though in no one primary shall he receive more than one party ballot.
Mass. Gen. Laws ch. 53, § 38