In Bereishis 1:29 we read:
And G-d said, Behold I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, on which is the fruit yielding seed; to you it shall be for food.
The Or HaChaim HaKadosh (ad loc.) explains, based on the Midrash (Bereishis Rabbah 21:7), that if Adam would have waited until Shabbos, he would have been allowed to eat from the tree of knowledge:
[...] they, of blessed memory, have already stated (Bereishis Rabbah 21:7) that if [Adam] had waited until the eve of Shabbat, he would have sanctified with wine [of that fruit] - so far [their words]; and from their words, you learn that [this] prohibition was not [to be] forbidden forever.
This is also quoted in sefer Magid HaRakiah Al HaTorah, Bereishis, by Rav Daniel Glatstein shlita (Parshas Bereishis, p. 86-87).
One opinion is that by Shabbos, the tree of knowledge would have been elevated above the concept of death.
Another opinion is given by the (Chasam Sofer Al HaTorah, Parshas Vezos HaBeracha) (although I don't understand that explanation). This explanation seems also to go against the lesson that Hashem wanted to teach us knowledge, and not that we've gotten it via the tree of knowledge, per Rabbi Dovid Tzvi Hoffmann. Refer also to the Chasam Sofer Al HaTorah here. Another possibility is given by Rav Shmuel Engel, known for his She'eilos U'Teshuvos Maharash Engel. He writes that according to halacha, nothing is considered to be in existence until it reaches a third of its way through (cited by Rav Glatstein in The Light And The Splendor, p. 361).
I am, however, looking for more explanations on why, if Adam would've waited until Shabbos, the fruits from the tree of knowledge would have been permited for him to eat from.