Rockport bills for taxes on a semi-annual basis (twice per year) within the fiscal year.
The first half of taxes (covers July 1st to December 31st) are typically due in the late fall/early winter. The 1st half taxes cannot be issued until the Commonwealth of Massachusetts has certified the tax rate for the Town.
The 2nd half taxes (covers January 1st to June 30th) are always due and payable by May 1st of the same fiscal year, i.e. 1st half taxes could be due December 21st, and your 2nd half taxes are due on May 1st.