The Torah discusses the dinim of the moadim in three different parshiyos: Emor, Pinchas, and Re'eh. In Iarshas Emor we read about the issue of melocha. In Parshas Pinchas we learn about the korbanos musafim, and in Parshas Re'eh we read about the particular mitzvos that are unique to each of the yomim tovim.
In the second half of Parshas Pinchas the Chumash lists off all the various yomim tovim, and includes Chag ha'Pesach and Chag ha'Matzos as two different yomim tovim. The afternoon of Erev Pesach is the time of hakorovas korban Pesach, which constitutes a yom tov known as Chag ha'Pesach. What we call Chag ha'Pesach, but the Torah refers to as Chag ha'Matzos, starts on the fifteenth of Nissan and lasts for seven days. Even though the Chumash only mentions the mitzvah of simcha in connection with Sukkos, the Torah she'b'al peh has established that it applies to all Shalosh Regalim, and even to Rosh Hashanah and Yom Kippur. Regarding Chag ha'Pesach, i.e. erev Pesach after chatzos, there is a dispute amongst the rishonim whether there is a chiyuv simcha. The din is well known that if one is in the midst of aveilus or shiva when yom tov arrives, the chiyuv simcha of yom tov cancels the remainder of shiva. If one is in the middle of sheloshim when yom tov arrives, then yom tov cancels the remainder of sheloshim. Because we have a rule that whenever there is a slight safeik in hilchos aveilus we follow the lenient position, the Remah (Yoreh Deah) paskens that when chatzos arrives on erev Pesach, the yom tov of Chag ha'Pesach cancels shiva or sheloshim and there is no need to wait until Chag ha'Matzos arrives.
The mishna tells us that although one does not observe aveilus on chol hamoed, nevertheless, one does rip kriya. The gemorah explains this is true because kriya is not part of the laws of aveilus. Pursuant to this, while the accepted opinion in Shulchan Aruch is that unlike other mitzvos, we are not mechanech children before bar mitzvah in the observance of aveilus, nonetheless there is chinuch for the mitzvah of tearing kriya, because kriya is not an element of nihug aveilus. The gemorah even says that if the one who lost a relative is an infant so young that he does not even understand anything about death, we tear a little bit of a kriya just to demonstrate to the menachamim what a tragedy has occurred.
After the destruction of the second Beis Hamikdash, the tanaim introduced three levels of ripping kriyah upon witnessing different aspects of the churban: kriya on the Beis Hamikdash in the state of churban, on Yerushalayim in the state of churban, and on arei Yehudah in the state of churban. The Magen Avraham points out that these three forms of kriya are unlike the law of kriya when a person dies. These are an aspect of aveilus, and therefore would not apply on chol hamoed or to a child before bar mitzvah.
Erev Pesach in the afternoon has the same din with respect to aveilus as chol hamoed. Therefore, if one visits the Kosel ha'Maaravi on erev Pesach in the afternoon, he would not tear kriya. Based on this din, many have the mistaken impression that on every erev Shabbos and on every erev yom tov after chatzos, one would not tear kriya when visiting the Kosel, but this is clearly a mistake. The Pischei Teshuva in the very end of Yoreh Deah mentions that many have the practice that on every erev Shabbos and every erev yom tov after chatzos, one does not observe shiva. This, however, is clearly a mistake and a carryover from erev Pesach after chatzos.