The last day to register to vote in time to cast a ballot in next month’s special election for US Senate is June 5, when the town clerk’s office will remain open until 8 p.m. to accommodate last-minute applications. Absentee ballots for the June 25 election, which will fill the seat left vacant when John F. Kerry was appointed US secretary of state, are available in the clerk’s office in Town Hall, and must be returned no later than noon June 24. Voter registration forms and absentee ballot applications are also available online at www.sec.state.ma.us. Sample ballots for the election, which pits Republican Gabriel Gomez against Democrat Ed Markey, are available online at www.natickma.gov. For more information, call the clerk’s office at 508-647-6430.