What an interesting and pertinent question! It’s true that the Festivals occur on specific calendar dates and we all have a calendar. Nevertheless, observing a “Second Festival Day” outside of Israel is a rabbinical decree and mitzvah and is a “safety measure” as I will explain.
In Jewish tradition the calendar is determined by eyewitness sightings of the new moon. The Jewish Supreme Court, the Sanhedrin in Jerusalem, was the authority for declaring when the new month began. In the days of the Temple, people outside of Israel simply would not know if the previous month was twenty-nine or thirty days, and would be in doubt as to the correct night of the Passover Seder and other Festivals as well. Since it generally took more than two weeks for messengers to get from Jerusalem to the Diaspora, people outside of Israel would need to keep two Seders out of doubt.
Nowadays, although have a fixed calendar, our Talmudic Sages decreed that the people outside of Israel continue to keep two days. In the words of the Talmud, “We continue the custom of our ancestors”. The reason for this decree is that if calendar communications and calculations “collapse” for any reason, people will still always be safely observing the Festivals. In addition, this decree shows the dependence of the world on the "word of God coming from Jerusalem". (Isaiah 2:3)
