In Tradition, the sabbat dates fall upon a date during which the Sun is in a certain degree of a specific astrological house. While the astrological dates might be more technical, what with having the planetary layout and subsequent astrological influences to one's advantage, neither date of celebration is considered "wrong".
Following is the astrological date chart, and below this chart is a four-year look ahead, at the astrological dates of each sabbat.
Samhain: Sun is at 15° Scorpio
Yule: Sun is at 1° Capricorn
Imbolc: Sun is at 15° Aquarius
Ostara: Sun is at 1° Aries
Beltaine: Sun is at 15° Taurus
Litha: Sun is at 1° Cancer
Lammas: Sun is at 15° Leo
Mabon: Sun is at 1° Libra
2008
Imbolc 2008: February 4
Ostara 2008: March 21
Beltaine 2008: May 5
Litha 2008: June 22
Lammas 2008: August 7
Mabon 2008: September 23
Samhain 2008: November 7
Yule 2008: December 22
2009
Imbolc 2009: February 4
Ostara 2009: March 22
Beltaine 2009: May 6
Litha 2009: June 23
Lammas 2009: August 8
Mabon 2009: September 24
Samhain 2009: November 8
Yule 2009: December 23
2010
Imbolc 2010: February 4
Ostara 2010: March 22
Beltaine 2010: May 6
Litha 2010: June 23
Lammas 2010: August 8
Mabon 2010: September 24
Samhain 2010: November 8
Yule 2010: December 23