A Partial Day is Regarded as an Entire Day

We have learned in the following braisa (Daf Yomi: Nazir 15a): One who buried his dead three days prior to the festival, the decree regarding the seven days of mourning are cancelled. One who buried his dead eight days prior to the festival, the decree regarding the thirty days of mourning are cancelled. He may take a haircut on the eve of the festival; if he did not, he is forbidden from taking a haircut after the festival. Abba Shaul disagrees and holds that he would nevertheless be permitted to take a haircut after the festival since in the same manner that three days of mourning prior to the festival cancels completely the seven days of mourning, seven days of mourning prior to the festival cancels completely the thirty days of mourning.

What is Abba Shaul’s reason? Is it not because he maintains that part of the day is like the entire day and the seventh day counts for the last day of shiva (seven days of mourning) and for the first day of the sheloshim (thirty days of mourning).

The Gemora asks that perhaps Abba Shaul only ruled in this manner by the halachos of shiva, which is only Rabbinical, but with respect to a nazir, where his halachos are of Biblical origin, he would not rule this way (that one day can be counted as two).

*** Tosfos (Moed Katan 19b) cites Harav Yom Tov that since we have established that part of the seventh day counts for the last day of shiva (seven days of mourning) and for the first day of the sheloshim (thirty days of mourning), a mourner would be permitted to take a haircut on the twenty-ninth day, since the seventh day counts as two days. He then cites a dissenting opinion that with respect to the halachos of sheloshim, we do not rule that the seventh day counts as two days.

*** Reb Elchonon Wasserman in Koveitz Heoros (39:3) explains the dispute between the Tanna Kamma and Abba Shaul in the following manner: Abba Shaul holds that a partial day is regarded as a full day, and therefore a day can be split into two, and it may be counted as two days. The Tanna Kamma, however, holds that one who has observed the halachos of the day can be regarded as if he observed them for the entire day, but the day itself cannot be regarded as two days.

*** The Rosh holds that this halacha that part of the day is like the entire day is even applicable at night. If one observed the halachos of mourning on the night of the seventh day, it should be regarded as if he observed them the entire day. He cites a Rashbam, who says that the custom is for the mourning period to conclude by day. The Rosh does not understand the reason for this.