Reference for Bekhorot 97:27

הוא לפדות ובנו לפדות הוא קודם לבנו רבי יהודה אומר

All the authorities concerned agree that as regards the laws of mourning the thirtieth day is counted as being like the previous day,<span class="x" onmousemove="('comment',' So that if the offspring died on the thirtieth day, the mourning ceremonies need not be observed by the father, as one can say that it was an untimely birth. Var. lec. (v. R. Gershom) have the following version: 'The thirtieth day is considered like the day after' i.e., there is no prohibition of washing one's clothes or cutting the hair.');">²⁵</span>

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