This is like he said above (27:3), that Yitzchak asked Esav to find and offer Pesach, which is eaten at night. Perhaps a support is "ha'Lechem," which implies that it is mentioned elsewhere ("ba'Erev Tochlu Matzos" - Shemos 12:18). However, refer to 27:3:1:2*. On Erev Pesach, Chametz is forbidden mid'Oraisa after midday, and mid'Rabanan after a third of the day, and Matzah is forbidden the entire day! Chachamim used to eat in the sixth hour; Chazal looked down on people who ate before four hours of the day (Shabbos 10a)! Perhaps Yitzchak was different; he ate early to reach Devekus (refer to 27:4:1:4), or in order to get the Torah Mitzvah of Birkas ha'Mazon. Or, he ate Matzah not made l'Shem Mitzvah; it is permitted the entire day (Pesachim 40a).