I found a useful US Navy page with the exact dates of the equinoxes and solstices for several years to come.
Imbolc / Candlemas : February 1st/ 2nd
Spring Equinox : March 20th (2010) / 21st (2011)
Beltane / May Eve : April 30th / May 1st
Midsummer : June 21st (2010 and 2011)
Lammas/ Lughnasadh : July 31st/ August 1st
Autumn Equinox : September 23rd (2010 and 2011)
Samhain / Halloween October 31st/ November 1st
Yule : December 21st (2010) / 22nd (2011)
Samhuinn : October 31-November 1
Winter Solstice (Alban Arthan or Alban Arthuan): Dec 21st (2010) / 22nd (2011)
Imbolc : February 1-2
Vernal Equinox (Alban Eiler or Alban Eilir): March 20th (2010) / 21st (2011)
Beltaine : April 30-May 1
Summer Solstice (Alban Heruin or Alban Hefin): June 21st (2010 and 2011)
Lughnasada : July 31-August 1
Autumn Equinox (Alban Elued or Alban Elfed): September 23rd (2010 and 2011)
Different Heathen communities and individuals celebrate different cycles of seasonal holidays based on their cultural affiliations, local traditions, and relationships with particular gods. There is no fixed calendar of Heathen festival dates. The three Heathen festivals most commonly celebrated in the UK are Winter Nights - usually celebrated in October or November, Yule - a twelve day festival that begins around the time of the winter solstice, and a festival for the Anglo-Saxon goddess Eostre in the spring.
Religio Romana festivals
There are loads of these; and different practitioners seem to celebrate different ones.