Reference for Bekhorot 115:35

ותיפוק ליה דבעינן למעבד הכירא בחדש וישן

And yet another reason is because it is a Holy Day, and you cannot tithe on a Holy Day on account of the required marking of the tenth animal with paint.<span class="x" onmousemove="('comment',' And painting on a Holy Day is work which is forbidden. And the reason why it was fixed on the twenty-ninth day of Elul and not on the thirtieth is because in the majority of years Elul is defective i.e., twenty-nine days, and thus the thirtieth day of Elul would be Rosh Hashanah.');">²³</span>

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