Or HaChaim on Numbers 28:26/אור החיים על במדבר כ״ח:כ״ו

1 בהקריבכם…בשבעתיכם, when you offer on your festival of weeks, etc. The plural indicates that this festival is not only meant for bringing the offering from the new wheat harvest, but that at the same time it also symbolises the completion of the count of the seven weeks commencing with the second day of Passover. This represents a major rehabilitation of our souls as explained by the Kabbalists. The Torah is careful to use the possessive suffix to indicate that the seven weeks represent the essential elements which distinguished the seven righteous people who between them form the spiritual foundation of the Jewish people. This is explained in detail in Tikkuney Hazohar chapter 21.

2Another reason why the Torah wrote בשבעתיכם is to disabuse the fools who thought that the words ממחרת השבת in Leviticus 23,25 refer to the Sabbath of Bereshit, i.e. the ordinary Sabbath, rather than the first day of Passover (compare Menachot 65). By saying "your weeks," the Torah makes it plain that the count started on a day which was subject to the terrestrial court to determine. The terrestrial court has no authority to change the date of the Sabbath, although it may postpone the day on which we observe new moons. We have a Baraitha in Menachot 72 where Rabbbi Yehudah also deduces the same ruling from the words in Deut. 16,9 תספר לך, "count for yourself." This excludes days over which our courts have no authority and which all mankind counts alike.