Бамидбар 33:3 Комментарий: Or HaChaim, Rabbeinu Bahya, Chizkuni & Rav Hirsch

ויסעו מרעמסס, They journeyed from Raamses, etc. The Torah had to mention twice that this journey occurred during the first month although it could have written "on the fifteenth," and I would have understood that the day mentioned was the first month. The Torah could also have written: "they travelled on the fifteenth of the first month after the Passover;" Perhaps the Torah alluded to what we have been told in Shemot Rabbah 15,11 that the month of Nissan is the month in which the zodiac sign of Israel is at its zenith and that the most auspicious day during that month is the fifteenth of that month. The Torah writes ויסעו בחודש הראשון, to describe the month during which Israel's fortunes ride high and it adds that this occurred on the fifteenth of that month, i.e. when the moon is full. The Torah added once more that it was in the first month, as the significance of the full moon is meaningless for Israel unless it occurs during the first month. The combination of these two factors was meaningful, was a good omen.

ממחרת הפסח, "on the day following the Passover-offering" The reference is not to the festival but to the day on which the lamb was slaughtered in anticipation of the Exodus. This occurred on the afternoon of the 14th of Nissan; the Israelites left Egypt on the morning of the 15th.

ויסעו מרעמסס בחודש הראשון בחמשה עשר יום וגו, “they began their journeys from Rameses on the fifteenth day of the first month, etc.;” the people had not been warned about the laws of Yom Tov [such as the prohibition of festivals regarding travel beyond the boundaries of their town except as regards the first night, when they had been forbidden to leave their houses on pain of death] except to abstain from eating chametz on that night.