This Pasuk is not in the Parsha of Korban Pesach. The Pesukim immediately preceding this one are taking about Matzah. The Parsha in general begins with Bechor.
The stages of Rosh Chodesh, during the day, and night are very understandable. Moshe Rabbeinu informed them about the Korban Pesach on Rosh Chodesh, and so it may follow that we should begin speaking to our children from then as well. And so ir says, ביום ההוא. The 14th day is when we are fully preoccupied with the Korban Pesach, and so perhaps we should talk about it then. That’s where זה comes in, to specify that only when this, the Matza is present.