Many of the names of the old holidays are of Celtic origin; and many of them have been adopted/converted into Christian holidays as part of the conquest of Paganism by the Church (for instance, Eostre becomes Easter, Yule becomes Christmas, etc.).. It was easier to adopt and re-interpret the traditions of the common people than to try to ban their holy days and traditional festivals all together. Reformers even went so far as to destroy sacred groves and temples to put up their chapels in the same locations.
Sabbats occur about every 6 weeks. Esbats occur when needed, and usually on the New or Full Moon. The 4 old festivals (Imbolc, Beltane, Lughnasad, Samhain) are celebrated by most of Pagandom. They are called the Cross Quarters. The Spring/Autumn Equinox and Summer/Winter Solstice celebrations are a more recent addition and are called the Quarters (not to be confused with the directional Quarters, East, South, etc.).
Pagans in the Southern Hemisphere may choose to celebrate the holy days in reverse, keeping Beltane in October and Samhain in May (for example). Let the inherent energy of the seasons in your area be your guide.
