I often mention Celtic natural seasonal festivals. I like mentioning them because people of European background may not know of their existence.
Celtic Calendar of Seasonal Festivals
More about the Celtic Calendar: http://en.wikipedia.org/wiki/Celtic_calendar
More about Imbolc (February 2) http://en.wikipedia.org/wiki/Imbolc
More about Beltane ( May 5) http://en.wikipedia.org/wiki/Beltane
More on Lughnasadh (August 1) http://en.wikipedia.org/wiki/Lughnasadh
More on Samhain (November 1) http://en.wikipedia.org/wiki/Samhain
The solstices and equinoxes can also be added to the Celtic seasonal cycle.
I also keep a calendar to enable me to track all those phenological events. Nature never sits still for a minute.
Kind Regards
Tony Powell