September 21st is the Autumn Equinox. Another old name for this day was Harvest Home before Aidan Kelly decided on the name Mabon.
In the mid 20th century, the United Nations decided to name September 21st as the International Day of Peace. I find this an interesting choice and a fascinating overlap. The first thought that came to mind was how the equinox is the balance point and the days surrounded it are more or less equal hours of day and night. A moment of peace between one the bright and long days of summer, and dark and long nights of winter in the Northern Hemisphere.
Whichever one you decide to observe, may you have a blessed day, and may these blessings continue to bring you more blessings in the future.