בחדש הראשון בארבעה עשר לחדש בין הערבים פסח... ובחמשה עשר יום לחדש הזה חג המצות לד'
Rashi explains that although the Festival actually begins in the evening (of the fifteenth day of Nissan), the Pesach-offering is slaughtered during the afternoon of the fourteenth. In the Torah, the word Pesach is used to describe the offering, while matzos – Chag HaMatzos, defines the Festival. Haamek Davar considers the time of Bein Ha’Arbayim, in the afternoon, to be part of the holy convocation of the Festival, so that labor is forbidden at that time. Why is there a separation between the time of slaughtering the sacrifice and the time that it is eaten – during the actual Festival?…