Mass. Gen. Laws ch. 59 § 58
In any city or town that accepts the provisions of this section, on all property taxes assessed in a city or town and collected by a city or town tax collector, discounts for the early and full payment thereof shall be three percent if the taxpayer pays the entire tax bill for the fiscal year in which said tax is due by November first or within thirty days if the bill for such tax was mailed later than October first.
In any city or town that accepts the provisions of this section and that has adopted the provisions of section fifty-seven C, the discount for early and full payment of the tax bill shall be three percent if the taxpayer pays the entire tax bill for the fiscal year in which said tax is due, notwithstanding any provisions of said section fifty-seven C, by August first or within thirty days if the tax bill was mailed later than July first. In the event it is determined at the end of a fiscal year that a taxpayer, who has exercised this option, has overpaid his tax for said fiscal year, the tax bill for the next fiscal year for said taxpayer shall be reduced by the amount that was determined to have been overpaid in the previous fiscal year.
Mass. Gen. Laws ch. 59, § 58