When you've missed the boat there's nothing you can do but wave to the passengers. If the train has already left the station, you might as well sit down and wait for the next one to arrive. There are many things in life that depend on being in the right place at the right time; if you're late, you've missed that opportunity forever.
Likewise, the Torah tells us that there are specific times for doing specific mitzvot. There is a proper time to put on tefilin, a proper time to light Shabbat candles, a proper time to eat matza, etc.
Thus, the Torah's narrative about Pesach Sheini - the "Second Passover" (which is always on the fourteenth of Iyar, a month after the first) expresses a very radical concept in Judaism.
Right before their Exodus from Egypt, G-d commanded the Jewish people to offer the Passover sacrifice on the fourteenthh of Nissan. One of the requirements was that a Jew had to be in a state of ritual purity. As a result, not everyone was permitted to bring an offering, and the Jews who were excluded felt terrible. "Why should we be left out?!" they demanded of Moses. They were so eager to observe the mitzvah that G-d relented, granting them another opportunity to bring an offering one month later, on the fourteenth of Iyar.
This story reveals the unfathomable depths of the Jewish soul and the infinite power of teshuva, repentance. It teaches us that every Jew is so intimately connected to G-d that when he makes a sincere and heartfelt demand, it "forces" G-d, so to speak, to open up new channels through which to send us His abundant blessings.
As the Previous Rebbe explained, the lesson of Pesach Sheini is that it is never too late to correct the past and return to G-d. It also emphasizes the power of a Jew's initiative. When a Jew cries out, from the depths of his soul and with a genuine desire to fulfill G-d's will, G-d listens to his plea and grants his request.
Ultimately, Pesach Sheini teaches us that we must never despair or give up, especially in bombarding G-d with our demand that He send us Moshiach immediately.
It is customary to mark this day by eating matzah, shmurah matzah if possible.