Moed Katan 19B

והתניא: הקובר את מתו שלשה ימים קודם לרגל ⁠— בטלה הימנו גזרת שבעה, שמונה ימים קודם לרגל ⁠— בטלה הימנו גזרת שלשים, ומגלח ערב הרגל. אם לא גילח ערב הרגל ⁠— אסור לגלח אחר הרגל.

But isn’t it taught in a baraita: In the case of one who buries his dead relative 3 days before a pilgrimage Festival, the decree of the 7-day period of mourning is nullified for him. If one buries his dead relative 8 days before a Festival, the decree of 30 days is nullified for him, and therefore he may cut his hair on the eve of the Festival. If, however, he did not cut his hair on the eve of the Festival, he is prohibited from cutting his hair after the Festival until the end of the 30-day period of mourning.

אבא שאול אומר: מותר לגלח אחר הרגל, שכשם שמצות שלשה מבטלת גזרת שבעה, כך מצות שבעה מבטלת גזרת שלשים.

Abba Shaul says: Even if he failed to cut his hair on the eve of the Festival, he is permitted to cut his hair after the Festival, for just as his observance of the mitzva of 3 days of mourning prior to the Festival cancels the decree of the 7-day period of mourning, and so if the deceased was buried 3 days before the Festival, the mourner is no longer required to continue this period of mourning after the Festival, so too, his observance of the mitzva of 7 days of mourning before the Festival cancels the decree of the 30-day period of mourning.

שבעה? והאנן שמונה תנן! קסבר אבא שאול: מקצת היום ככולו, ויום שביעי עולה לו לכאן ולכאן. אמר רב חסדא אמר רבינא בר שילא: הלכה כאבא שאול.

The Talmud asks: Why does Abba Shaul speak of the observance of 7 days of mourning? But didn’t we learn in the Mishnah that it requires 8 days? The Talmud explains: Abba Shaul maintains that the legal status of part of the day is like that of an entire day, and therefore the 7th day counts as both the final day of the 7-day period of mourning and the 1st day of the 30-day period of mourning. Rav Ḥisda said that Ravina bar Sheila said: The halakha is in accordance with the opinion of Abba Shaul.