Tosfos asks: Why didn’t the Gemora cite the verse where Yosef mourned for his father Yaakov in Breishis [50:10]: And he made for his father a mourning of seven days?
Tosfos answers: That verse is referring to the period of mourning prior to the burial.
It is noteworthy that the Ibn Ezra on that verse comments that the mourning mentioned here is referring to the period after the burial. Rabbi Aharon Levine in his sefer Hadrash V’haeyun wonders why the Ibn Ezra explained the verse in that manner, when it is evident that our Gemora does not learn that way. The Rif (Brochos) does derive the laws of mourning from Yaakov.
Tosfos cites a Yerushalmi that answers differently: We cannot derive the laws of mourning from the death of Yaakov because that was prior to the giving of the Torah.
This seems a bit perplexing because there are many halachos (time for the Minchah prayer) that are derived from verses in the Torah even though the incidents transpired prior to the giving of the Torah?
The Chasam Sofer answers: It is written regarding the laws of mourning ‘Banim atem,’ You are My children. When did Klal Yisroel get the status of being children of Hashem? After receiving the Torah. It is for this reason that the laws of mourning must be derived from a verse which occurred after the Torah was given.
Rav Elyashiv answers: While it is true that we could have learned many of the laws of mourning from Yaakov and Yosef, there are some laws that we would be incapable of deriving from there. A mourner cannot study Torah, nor can he don tefillin on the first day and before the burial he cannot perform any mitzvos. If the source of the laws of mourning would be from Yaakov and Yosef, we would say that those laws were included in the mourning period for the mourners who did not have these obligations (the Patriarchs before the Torah was given); however we, who have an obligation to study Torah, don tefillin and perform mitzvos, cannot be subject to these prohibitions. It was for this reason that the Gemora found the source for the laws of mourning from a verse in Amus, after the Torah was given.
[Once again, my anonymous friend pointed me in the right direction.]