Halakhah sobre Exodo 12:18

I also found explicit with regard to the first of the holidays (Passover), which God gave to Israel prior to instructing them about the Sabbath, “on the fourteenth day of the month at evening you should eat unleavened bread, until the twenty first day of the month at evening” (Exodus 12:18), a total of “seven days” (ibid. 12:19). Thus the evening of the fifteenth is the first day. It is also written “[neither shall any of the flesh] from which you offered in the evening of the first day [be left over] until the morning” (Deuteronomy 16:4). Also, it is known that the firstborn were smitten at midnight (Exodus 12:29), yet it is written “on the day that I smote all firstborn” (Numbers 3:13, 8:17).⁴ The verse informs us that God sanctified all Jewish first born on the day that the Egyptian first born were slain. It seems likely that this took place on the first day of Passover. Also in Scripture “this day is a day of tidings…if we wait until the morning light” (2 Kings 7:9).

The commandment of eating matsah: To eat matsah-bread that is made from a species of grain on the night of the fifteenth of Nissan (Pesachim 35a), as it is stated (Exodus 12:18), "in the evening, you shall eat matsot." And the understanding [of "evening"] is the night of the fifteenth of Nissan - whether it be at a time when the Pesach sacrifice is present or whether it be at a time that it is not present.