"These are HaShem's appointed festivals, holy convocations, which you shall proclaim in their appropriate 
Here's a question that hardly one Jew in a hundred will be able to answer correctly: What is the first festival after Pesach?
seasons: In the first month [Nissan], on the fourteenth of the month in the afternoon is Pesach for HaShem. And on the fifteenth day of this month is the Festival of Matzot for HaShem; for seven days you shall eat matzot. On the first day you will have a holy convocation, you will do no laborious work, and you will sacrifice a burnt-offering to HaShem for seven days. On the seventh day is a holy convocation, you will do no laborious work." (Leviticus 23:4-8)
Here's a question that hardly one Jew in a hundred will be able to answer correctly: What is the first festival after Pesach? Take a few moments now to think of your own answer. Well? What is the first festival after Pesach?
The instinctive answer, that almost every Jew will give, is Shavuot. Sorry, wrong answer. Others might think of Lag BaOmer, Yom HaAtzma'ut, or the seventh day of Pesach (shvi'i shel Pesach).
Still not the right answer. Think about it for another few moments and if necessary re-read the above passage from Leviticus, which constitutes part of the Torah reading for the second day of the festival.
Pesach is on the fourteenth of Nissan, the day on which we are commanded to offer the Pesach-sacrifice; and the first festival after it is Chag HaMatzot, the Festival of Matzot, which lasts for seven days, from the fifteenth to the twenty-first. It is popularly called Pesach (or Passover), but this is an inaccurate colloquialism. In the prayers, it is always called Chag HaMatzot - which is its rightful name.
The Torah gives the reason for the Festival of Matzot: "This day will be your day of remembrance and you will celebrate it as a festival to HaShem for all your generations; as an eternal decree shall you celebrate it. For seven days you will eat matzot.... So you will safeguard the matzot, because on this very day I have taken your legions out of the land of Egypt, so you will safeguard this day throughout your generations as an eternal decree." (Exodus 12:14-17)
We all know the story perfectly well: "So they baked the dough that they had brought out of Egypt into cakes of matzot because it had not leavened, because they had been driven out from Egypt, and they could not wait; neither had they brought any provisions with them." (verse 39)
These matzot that we brought with us out of Egypt would last us for the first month in the Sinai Desert (see Rashi on Kiddushin 38a, s.v.chaseir shloshim):
"They travelled from Elim, then they came - the entire congregation of the Children of Israel - to the Wilderness of Sin, which is between Elim and Sinai, on the fifteenth day of the second month from their Exodus from Egypt. And the entire congregation of the Children of Israel complained against Moshe and Aaron in the wilderness. The Children of Israel said to them: 'If only we had died at HaShem's hand in the land of Egypt, while we were dwelling by the flesh-pots, when we were eating bread to satiety! But you have brought us out to this wilderness to put this entire assembly to death by starvation.' And HaShem said to Moshe: 'Behold! I am about to rain food from heaven down on you.' " (Exodus 16:1-4)
This food from heaven was what the Israelites called mahn (manna): "When the layer of dew rose, behold! there was on the surface of the wilderness something thin and wafery, thin as the frost on the ground. And the Children of Israel saw, and they said to one another: 'Mahn hu,' because they did not know what it was." (verses 14-15)
The phrase mahn hu could mean, "This is preparation for food" (Rashi, Metzudat Zion to Psalms 68:24); or, "What is this?" (Rashbam); or, "It is food" (Ibn Ezra); or, "It is a gift" (Rabbi Shimshon Raphael Hirsch).
In any event, "the House of Israel called it mahn [manna]" (Exodus 16:31); "and the Children of Israel ate the mahn for forty years, until their arrival in a settled land; they ate the mahn until their arrival at the edge of the Land of Canaan." (verse 35)
The Talmud (Kiddushin 38a) sees a difficulty in this verse: "Did they really eat it for forty full years? Actually they ate it for forty years less thirty days." The calculation is simple (see Rashi on Exodus 16:35): the mahn began falling for the Children of Israel on the 16th of Iyar. Forty years later, they entered the Land of Israel on the 10th of Nissan (Joshua 4:19); then "the Children of Israel encamped in Gilgal, and they carried out the Pesach sacrifice on the fourteenth day of the month in the evening, in the plains of Jericho. And they ate of the grain of the Land on the day after the Pesach sacrifice, matzot and roasted grain, on this very day. And the mahn desisted the next day, when they ate of the grain of the Land; there was no more mahn for the Children of Israel, and they ate of the produce of the Land of Canaan in that year." (Joshua 5:10-12) 
The matzot were the food that we ate for a month after leaving Egypt.
Hence, the mahn stopped falling on the 16th of Nissan - forty years less thirty days after it began. The Talmud (ibid.) resolves this apparent contradiction: "This tells you that the taste of the cakes which they had brought out of Egypt was the taste of mahn."
This makes the mitzvah of eating matzot on Pesach far more powerful than merely eating crackers for a week. The matzot were the food that we ate for a month after leaving Egypt - and they had the taste of manna, no less. Chag HaMatzot, the Festival of Matzot, is our annual celebration of eating the original matzot that we had baked while still in Egypt - and which had the taste of the manna with which God would feed us for almost forty years in the desert while travelling to Israel.
And the manna itself, holy and spiritual though it certainly was, was but a substitute for the produce of the Land of Israel. And the produce of the Land of Israel, the truly holy and spiritual food, is what we begin to celebrate on the second day of Chag HaMatzot, when we begin to count the omer - the beginning of the harvest.
What a truly awe-inspiring thought to chew over while chewing the matzot.