The delivery of the materials ended up taking 2 months, so we were delayed to December 12.
Literal
Material [attributive-の] delivery-date [subject-が] 2-months took-[unfortunately-てしまった][because-ので], December 12 [to-に] was-delayed-[unfortunately-てしまいました].
Business correspondence. 納期 ('delivery deadline, delivery date') is a standard business term combining 納 ('delivery') + 期 ('period, term'). ~てしまう is used twice to mark two regrettable outcomes. ~ので ('because') is a slightly more polite/formal alternative to から.