“ And seems to me that the custom of the elders was that they take one of their young sons to serve them, and he would lean on his arm and would never be parted from him, and he called him "a son of his old age", for he served him in old age, and here Jacob took Joseph for this thing and was always with him, and therefore he did not work with the sheep when they grazed in far away places. ”

“ This was a signal that he [Yosef] will be the leader in the field and in the house, as in the matter of "and I will dress him in your tunic" (Isaiah 22:21) , and this is like our sages, of blessed memory, said (Bava Kama 11a) , "the eldest of the heirs, as it is in the interest of them all that his words should be respected." לְאות שֶׁיִּהְיֶה הוּא הַמַּנְהִיג בַּבַּיִת וּבַשָּׂדֶה, כְּעִנְיָן "וְהִלְבַּשְׁתִּיו כֻּתָּנְתֶּךָ" (ישעיהו כב, כא) , וּכְאָמְרָם זִכְרונָם לִבְרָכָה: "בִּגְדול אֲחֵי כִּי הֵיכִי דְּלִשְׁתַּמְעָן מִלֵּיהּ" (בבא קמא יא, ב) . ”

“ Rashi And it was on a certain day וַיְהִי֙ כְּהַיּ֣וֹם הַזֶּ֔ה
This is saying that a special day had arrived, a day of laughter, their festival day everyone went to the house of idolatry. She, [the wife of Potifar] , said "I do not have a better day to fulfill my need with Joseph than today". She told them, "I am sick and I can not go". כלומר ויהי כאשר הגיע יום מיוחד, יום צחוק, יום איד שלהם שהלכו כולם לבית עבודה זרה, אמרה אין לי יום הגון להזקק ליוסף כהיום הזה. ”