By Marie Mayer
The dates for the beginning and end of months are based on the Gregorian calandar. If you go further back to the more pagan calandars, you'll find that the dates that are currently used for beginning/end of the seasons were actually the middle.
The beginnings were Samhain (October 31), Imbolc (February 2), Beltane (May 1), and Lughnasadh [Lammas Day] are the beginning.
The middle of the seasons are Yule/[Christmas], Ostara[Easter], Midsummer, and Mabon[Mid-Autumn]