The eight festivals
Four solar points (the two solstices and two equinoxes) alternate with four seasonal or 'cross-quarter' festivals, giving a festival roughly every six or seven weeks.
Where it comes from
The eightfold Wheel developed within mid-twentieth-century British Wiccan and Druid circles and is particularly associated with Gerald Gardner and Ross Nichols of the Order of Bards, Ovates and Druids. Individual festivals draw on genuinely older Gaelic and Germanic observances, but the eightfold wheel as a single calendar is modern.
Names came later still
Some sabbat names are attested seasonal festivals: Samhain, Imbolc, Beltane, Lughnasadh are recorded in medieval Irish sources. Others, notably Ostara, Litha and Mabon, were adopted as sabbat names in the twentieth century, Mabon only in the 1970s.
Why this matters
The Wheel doesn't need an invented ancient pedigree. It does mean that claiming an unbroken ancient lineage is inaccurate, and unnecessary.