I'm not sure what you mean by "basis" but I can tell you that Wiccans generally celebrate these days:
Samhain Lore (October 31st)
Samhain, (pronounced SOW-in, SAH-vin, or SAM-hayne) means "End of Summer", and is the third and final Harvest. The dark winter half of the year commences on this Sabbat.
Yule Lore (December 21st)
Yule, (pronounced EWE-elle) is when the dark half of the year relinquishes to the light half. Starting the next morning at sunrise, the sun climbs just a little higher and stays a little longer in the sky each day. Known as Solstice Night, or the longest night of the year, much celebration was to be had as the ancestors awaited the rebirth of the Oak King, the Sun King, the Giver of Life that warmed the frozen Earth and made her to bear forth from seeds protected through the fall and winter in her womb. Bonfires were lit in the fields, and crops and trees were "wassailed" with toasts of spiced cider.
Imbolc, (pronounced "IM-bulk" or "EM-bowlk"), also called Oimealg, ("IM-mol'g), by the Druids, is the festival of the lactating sheep. It is derived from the Gaelic word "oimelc" which means "ewes milk". Herd animals have either given birth to the first offspring of the year or their wombs are swollen and the milk of life is flowing into their teats and udders. It is the time of Blessing of the seeds and consecration of agricultural tools. It marks the center point of the dark half of the year. It is the festival of the Maiden, for from this day to March 21st, it is her season to prepare for growth and renewal. Brighid's snake emerges from the womb of the Earth Mother to test the weather, (the origin of Ground Hog Day), and in many places the first Crocus flowers began to spring forth from the frozen earth.
March 21 -- Ostara -- Spring or The Vernal Equinox
Also known as: Lady Day or Alban Eiler (Druidic)
As Spring reaches its midpoint, night and day stand in perfect balance, with light on the increase. The young Sun God now celebrates a hierogamy (sacred marriage) with the young Maiden Goddess, who conceives. In nine months, she will again become the Great Mother. It is a time of great fertility, new growth, and newborn animals.
April 30th -- Beltane
Also known as Roodmas or May Day
Beltane has long been celebrated with feasts and rituals. Beltane means fire of Bel; Belinos being one name for the Sun God, whose coronation feast we now celebrate. As summer begins, weather becomes warmer, and the plant world blossoms, an exuberant mood prevails. In old Celtic traditions it was a time of unabashed sexuality and promiscuity where marriages of a year and a day could be undertaken but it is rarely observed in that manner in modern times.
June 21 -- Summer Solstice -- Litha
Also known as: Alban Heruin (Druidic)
Although the name Litha is not well attested, it may come from Saxon tradition -- the opposite of Yule. On this longest day of the year, light and life are abundant. At mid-summer, the Sun God has reached the moment of his greatest strength. Seated on his greenwood throne, he is also lord of the forests, and his face is seen in church architecture peering from countless foliate masks.
July 31st Lughnasadh / Lammas
Lughnasadh means the funeral games of Lugh (pronounced Loo), referring to Lugh, the Irish sun god. However, the funeral is not his own, but the funeral games he hosts in honor of his foster-mother Tailte. For that reason, the traditional Tailtean craft fairs and Tailtean marriages (which last for a year and a day) are celebrated at this time.
Autumn Equinox, 2nd Harvest, September 21st
Mabon, (pronounced MAY-bun, MAY-bone, MAH-boon, or MAH-bawn) is the Autumn Equinox. The Autumn Equinox divides the day and night equally, and we all take a moment to pay our respects to the impending dark. We also give thanks to the waning sunlight, as we store our harvest of this year's crops. The Druids call this celebration, Mea'n Fo'mhair, and honor the The Green Man, the God of the Forest, by offering libations to trees. Offerings of ciders, wines, herbs and fertilizer are appropriate at this time. Wiccans celebrate the aging Goddess as she passes from Mother to Crone, and her consort the God as he prepares for death and re-birth.