The lunar year consists of 354 days. On average the first moon, or old moon, of the Penobscot calendar began on December 21st, but it could begin anywhere from December 5th to December 30. Every third year Penobscots added a 13th moon, the "inserted moon". Further adjustment was needed two or three times per century, when the 13th moon was inserted in a second year instead of a third year.
Learn the names of the Penobscot moons
Learn the days of the week in Penobscot