Rashi on Leviticus 23:39/רש"י על ויקרא כ״ג:ל״ט

1אך בחמשה עשר יום... תחגו BUT ON THE FIFTEENTH DAY… YE SHALL OFFER A חג — i. e., a sacrifice of peace-offerings as a חגיגה (the pilgrims’ sacrifice on the Festivals) (Chagigah 9a). Since it states that this has to be brought on the fifteenth day of the month, one might think that it (the offering of קרבן חגיגה) shall set aside the Sabbath (supersede the Sabbath law, if that day be the fifteenth of the month)! Scripture, however, uses the word אך (the particle that implies a limitation; cf. Rashi on v. 27): suggesting that it must not be offered on Sabbath because it has a supplementary period consisting of all the seven days of the Festival.

2באספכם את תבואת הארץ [ON THE FIFTEENTH DAY OF THE SEVENTH MONTH] WHEN YE GATHER IN THE INCREASE OF THE LAND — This tells us that this seventh month (in which סכות occurs) must be at the time of the ingathering. Hence we learn that they are enjoined to intercalate the years if necessary (Sifra, Emor, Chapter 15 6), because if there be no such intercalation it (the seventh month) would sometimes fall in the middle of the summer or of the winter.

3תחגו YE SHALL CELEBRATE (OFFER) the peace-offerings of the חגיגה.

4שבעת ימים SEVEN DAYS (during seven days), meaning that if one has not brought them on one he may bring them on another of the seven days. But I might think that the meaning is that one has to offer them on each of the seven days! Scripture, however, states (v. 41): וחגתם אתו, “and ye shall make it the day of the חגיגה” — this implies one day and no more. Why, then, does Scripture say “seven”? In reference to the supplementary period (Chagigah 9a).