Sifrei Mitzvot para sa Levitico 23:8

These six days on which Scripture prohibited labor—being the first and seventh days of Passover, the first and eighth days of Sukkot, the day of Shavuot and the first day of the seventh month (Rosh Hashanah)—are called holidays (yamim tovim). The [degree of] cessation of work is equal in all of them, in that all work is prohibited except for labor that is for the sake of eating—as it is stated (Exodus 12:16), "only anything that is eaten by any person."

... And they learned from the oral tradition that it is not Shabbat (that "from the morrow of the Shabbat" is not the weekly Shabbat) but rather the holiday [of Passover]. And so did the prophets and the high courts see in each and every generation, that they would wave the omer on the sixteenth of Nissan - whether it was on a weekday or on Shabbat. And behold, the Torah states (Leviticus 23:14), "Until that very day, you shall eat no bread or parched grain or fresh ears." And [in the Prophets, it] states (Joshua 5:11), "On the day after the Passover offering, on that very day, they ate of the produce of the country, unleavened bread and parched grain." And if you should say that this Passover came out on Shabbat - like the fools imagine - how could Scripture make the license to eat the new grain dependent on something which is not the main point nor the cause, but rather just happenstance. Rather, since it made it dependent on the morrow of Passover, it is a clear matter that the morrow of Passover is the cause of the license [to eat] the new grain; and we are not concerned about which day of the week it is.

And seventh of the Nissan [holiday, i.e. Passover]