The esbats are the most free-form and personalized of the two kinds. Since esbats are lunar days, they are generally observed as the full of the moon (though some observe the new moon, instead of or in addition to the full). Wiccans associate the moon with the Goddess, and thus esbats are generally held to reflect upon or honor her in some way, as well as to cast spells empowered by the current phase of the moon. Generally for a esbat I prefer to think of some manner in which the Goddess has been touching my life since the last full moon, and use it in some way during the ritual.

The sabbats are slightly more structured in that each has a part of a traditional story associated with it. The sabbats are observed as the 4 seasonal quarters (the equinoxes and solstices) as well as 4 days between them... Since these days are solar, they are associated with the God. They also tell the story of life, death and rebirth, in synch with the patterns of nature during that season. For example days in spring celebrate birth, in summer fertility, fall harvest and aging, late fall death, winter awaiting rebirth.. etc.

The Sabbats

This is the time at which the cycle of death (as represented by shortening days) changes to the cycle of rebirth (as represented by lengthening days).

colors: red, green (from the holly tree)

Imbolioc (aka Candlemas or Lupercus)
date: February 2 (some traditions observe the 1st or 14th)

The festival of lights, encouraging the return of the sun.

colors: white or green and white

Ostara
date: Spring equinox, circa March 21

The start of Spring, welcoming the start of the fertile season and plantings ahead

colors: white or light green, yellow and pink

Beltane (aka May Day)
date: April 30, or May 1

Spring is getting into full swing as plants are blooming, trees are leaving out etc. It is a celebration of fertility at it's peak.

colors: Red and White

Midsummer (aka Litha)
Date: Summer Solstice, circa June 21

The longest day of the year, symbolic of the God at the peak of his glory. It is a day of purification, and great magical power.

colors: White

Lughnasadh (aka Lammas)
date: August 1

Feast of the first harvest and thanksgiving

colors: Red and Orange

Mabon
Date: Autum Equinox, circa September 21 The end of summer and beginning of fall, the second harvest. colors: Red and Brown

Samhain (aka Halloween)
Date: October 31, or November 7

Honor of the dead, exploration of mysteries.

colors: Red, Orange and Black