TB Bekhorot 26b Definition please

The first part of the Mishnah on daf TB Bekhorot 26b discusses the laws of bekhorot, firstborn cattle and sheep outside the land of Israel and the Temple no longer stands. In this situation the laws of the bekhorot are no longer consider Torah laws, but only rabbinic prohibitions. “Until when must an Israelite tend to and raise a firstborn animal before giving it to the priest? With regard to a small animal, e.g., a sheep or goat, it is thirty days, and with regard to a large animal, e.g., cattle, it is fifty days. Rabbi Yosei says: With regard to a small animal, it is three months.” (Sefaria.org translation) When the Temple stood in the land of Israel, the unblemished firstborn may be offered up as a sacrifice on the altar any time after eight days. A blemished animal may be eaten by the kohen any time he wishes after the same eight day waiting period.

The Gemara wants to know where the numbers 30 and 50 come from. Rav Kahana derives these numbers based on an interpretation of two verse by connecting the redemption of the firstborn with sheep and the 50 days of the omer between Passover and Shavuot with oxen.

“Rav Kahana said: They are derived from a verse, as the verse states: “You shall not delay to offer of the fullness of your harvest, and of the outflow of your presses; you shall give to Me your firstborn sons. So you shall do with your oxen, and with your sheep; seven days it shall be with its mother, on the eighth day you shall give it to Me” (Exodus 22:28–29). The second part of verse 28 is juxtaposed to the second animal mentioned in verse 29. This juxtaposition teaches that just as a firstborn son is redeemed when he is thirty days old (Numbers 18:16), so too a sheep is given to the priest only when it is thirty days old.

“The verses also state: “You shall not delay the fullness of your harvest, and of the outflow of your presses…So you shall do with your oxen” (Exodus 22:28–29). Just as the first fruits are brought on the festival of Shavuot, fifty days after Passover, so too the firstborn oxen are given to a priest when they are fifty days old. Here, the first part of verse 28 is juxtaposed with the first animal mentioned in verse 29.” (Sefaria.org translation)

The Gemara challenges these numbers and ask why not 60 days, the first 30 days plus another 30 days? “The Gemara answers: The interpretation of the verse with regard to the exact amount of time was given only to the Sages (לֹא מְסָרְךָ הַכָּתוּב אֶלָּא לַחֲכָמִים.), and they determined that the owner must take care of his ox for fifty days.” (Sefaria.org translation) “Since the Torah itself did not reveal the length of this additional waiting period, we can conclude with certainty that it gave you over to the Sage’s, who will explain to you the reason {for waiting precisely 50 days}, and you should comply with their teaching(Rashi)” (Art Scroll edition, page 26b4, note 28)  The Gemara is teaching us that the rabbis are not legislating law. They are just defining what the verse means.

Another example of this definitional role of the rabbis is the intermediate days of a holiday, Khol Hamoed-חוֹל הַמּוֹעֵד). These days are not holy days, moed, nor are they regular weekdays, khol. Some work is prohibited and some work is permitted. The rabbis are not legislating only defining what work is prohibited and what work is permitted.