A great referral tool with the actual astral times and dates for all our high days!
Chart of 2012 equinox, solstice and cross quarter dates and times, worldwide from archaeoastronomy.com
At the bottom in their foot notes it says the Equinox and Solstice data is from the US Naval Observatory in Washington DC and the Cross Quarter moments are obviously the midway points between the solstices and equinoxes, but are measured scientifically by a former NASA scientist for this info. It includes southern hemisphere as well as northern hemisphere info.