Midrash su Deuteronomio 16:11

4) R. Yossi says: "from the morrow of the Sabbath": from the morrow of the festival. I might think, from the morrow of the Sabbath of creation. Now is it written On the morrow of Sabbath on Pesach? Is it not written only "from the morrow of the Sabbath"? Isn't the entire year filled with Sabbaths? Go and figure which Sabbath is meant! And, furthermore, it is written here "from the morrow of the Sabbath," and, below, (Devarim 16:11), "from the morrow of the (seventh) Sabbath." Just as there, the reference is to a time period (i.e., the end of the seventh week) conjoined with the beginning of a festival (Shavuoth); here, too, the reference is to a time period (the omer) conjoined with the beginning of a festival (Pesach, which begins with the fifteenth day of Nissan).

(Devarim 16:11) "And you shall rejoice before the L-rd your G-d": It is written here "rejoice," and elsewhere (Ibid. 27:7) "rejoice." Just as there, the rejoicing is with peace-offerings, here, too, it is with peace-offerings.