From the Londonderry Facebook page:

IMPORTANT INFORMATION ABOUT TAX BILLS: We know that many of you are awaiting your tax bill and may be wondering when it will arrive. As of today, tax bills have NOT been processed as the Town has not been assigned its tax rate yet from DRA (the NH Department of Revenue Administration). Once it is received and the bills are processed, the due date will be 30 days from the date the bills are mailed.