Mabon
The autumnal Equinox, falling around the 21st of September is the second day of the year where night and day are in balance.It is the time of year where The God gets ready to leave us and descent into the underworld, Ready to be re-born at Yule time.

Nature is in rapid decline now as the earth gets ready to rest over the winter. It is a time where we can walk out in the countryside to collect seeds to plant in the following spring. We find this to be a thoughtful time as the days become shorter and the nights longer.

I'm not sure if Mabon has an associated Christian day.