מדרש על ויקרא 23:9

3) If the year should not have been intercalated, and they intercalated it perforce or unwittingly or mistakenly — Whence is it derived that it is (nevertheless) intercalated? From (Vayikra 23:9) ("These are the festivals of the L–rd, callings of holiness, which you shall call (them) in their times": "them" — even unwittingly; "them" — even mistakenly; "them" — even perforce.