Reference for Bekhorot 115:28

זימנין דמחסרין ליה לאב ובעינן למיעבד הכירא בחדש וישן:

ON THE FIRST OF NISAN' in accordance with the opinion of R'Simeon B'Gamaliel who said: Two weeks.<span class="x" onmousemove="('comment',' I.e., fourteen days are the period of preparation for the Passover, these Tannaim not accepting the prescribed period of the Peras.');">¹⁹</span>

Jastrow

Jastrow

Jastrow