מדרש על ויקרא 23:39

5) (Vayikra 23:39) ("But on the fifteenth day of the seventh month, when you gather the produce of the land, you shall calibrate the festival of the L–rd seven days; on the first day a resting and on the eighth day a resting.") Beth Shammai say: I might think that the chagigah (olath re'iah [burnt-offering of "visitation"]) could be offered on the festival (itself); it is, therefore, written "But" (a term of exclusion). On the moed (the intermediate days of the festival) you offer the chagigah, but not on the festival (itself). Beth Hillel say: I might think that the chagigah could be offered on the Sabbath; it is, therefore, written "But." On the festival you may offer the chagigah, but not on the Sabbath.

6) "when you gather the produce of the land": Intercalate the year (if necessary) so that the festival occur when you gather the fruits.

7) R. Akiva says: What is the intent of (Vayikra 23:37) "These are the festivals of the L–rd which you shall call them holy callings"? If to the days of the festival (proper), these are already mentioned, viz. (Vayikra 23:39) "on the first day, a resting, and on the eight day, a resting. If so, what is the intent of "which you shall call them holy callings"? These are the intermediate days of the festival, on which (non crucial) labor is forbidden.