There are minor Jewish holidays of whom many Jews simply don't know that they exist. First of all, those holidays are not official and secondly, you necessarily won't find them on every calendar.
During the days of the Second Temple, the Jewish had many more holidays than we have today. Minor holidays which were forgotten after the Temple destruction. Talmud Shabbat 13b, for instance, is teaching us about a scroll called "Megillath Ta'anit" listing at least another 35 holidays compared to the amount we have today.
Today, Sunday (6 May), we celebrated "Pessach Sheni - the Second Pessach". Originally the day was thought of providing Jews who were impure during the first Pessach and thus unable to offer the Pessach sacrifice (Korban Pessach) with a second offering chance. The author of "Sha'arei HaMoadim", the third Lubavitcher Rebbe Menachem Mendel Schneersohn, 1789 - 1866, said that whoever is leaving out Pessach Sheni after being impure on the first Pessach, is doing so on purpose. Furthermore, the Rebbe stated that people converting to Judaism between the first Pessach and the second Pessach are thus able to sacrifice their Korban Pessach on Pessach Sheni. They are even obligated to keep the second Pessach.
Today we mostly eat a piece of Mazza in order to commemorate Pessach Sheni. The Rambam (Maimonides, 1135 - 1214) explained that Pessach Sheni is an independent holiday from the first one.
What do we learn from having a second Pessach ?
That there is always a second chance in life. If we fail for one or the other reason and it wasn't our fault then G - d may provide us with a second chance.
Rebbe Menachem Mendel Schneersohn (the third Lubavitcher Rebbe) stated that "Pessach Sheni" is an obligation only for those Jews who were unable of sacrificing their Korban Pessach on the first Pessach. Today it is for those who didn't have a Seder on the first night. However, you don't need to start a Seder neither have to clean the house "kasher le'Pessach".
The Chabad - book "Sha'arei HaMoadim" considers "Pessach Scheni" as a day of Teshuva (return to G - d). We receive the Torah on Shavuot and should therefore dig into ourselves and become aware of the purpose of our lives. How do we cause some Teshuva on Pessach Sheni ? By studying Torah !
The first Pessach is being celebrated in the Jewish month of NISSAN whereas Pessach Sheni is being celebrated in IYAR. In Nissan the upper spiritual worlds influence us in our physical world. On Pessach Sheni, however, it is us in the lower world to influence the upper worlds. Both Pessach holidays thus stand for a completion.
The Pessach sacrifice is only for Jews because the other nations were not Paraoh's slaves in Egypt (see Sha'arei HaMoadim on Pessach Sheni).