There are many celebrations that Witches observe, but they fall into one of two types. They are either Esbats, or Sabbats.
An Esbat is any ritual that does not fall on one of the eight Sabbats. These can be Full Moon Rituals, Dark Moon Rituals, New Moon Rituals, or simply the correct astrological time for a spell casting. Esbats are where Witches do their Magick.
Sabbats are any one of the eight celebrations around the year, that mark the change of season. As Witches, we recognize that the seasons play the biggest part in how we live our lives. Every culture has seasonal celebrations because the seasons are what made the human race what it is.