More than one reader noted that, in the piece I sent for Yom Kippur, I incorrectly mentioned the 10th of Nissan as the date of Passover. I’d like to explain the original idea that I was trying to describe (an idea that I may have read at least part of in one of Rav Mordecai Breuer's works).
In Egypt, just before the Exodus, God tells the people to begin their calendar with the month of Nissan. They are to set aside the Paschal lamb on the 10th of Nissan. Which is five days before Passover, the seven-day holiday that begins on the 15th.
Today our calendar begins not in Nissan, but with the month of Tishrei. Rosh HaShanah comes at the start of Tishrei. Yom Kippur comes on the 10th. And Sukkot, a seven-day holiday, begins on the 15th.
Obviously the dates of Tishrei -- our season of repentance and forgiveness -- run parallel to the dates Nissan, our season of freedom.
Perhaps because repentance and freedom are variations on a single theme.
Note that, outside of Israel, Passover and Sukkot are observed for one extra day.