If the orientation of the monuments that they left behind is anything to go by, the peoples of megalithic Britain observed both quarters (sunsteads and evendays) and cross-quarters (Samhain, Imbolc, etc.).
Just like we do.
Different peoples, different ways. As they've come down to us, the cross-quarters are largely a phenomenon of Keltic cultures, the quarters Germanic; hence the names by which they're generally called.
For this reason, some purists have decided to restrict themselves to observance only of quarters or cross-quarters. Well, everyone gets to make his or her own call. My own position is that purism is its own punishment.
According to maverick historian Stephen J. Yeates, the Anglo-Saxon tribe known as the Hwicce—the original Tribe of Witches—settled in the territory of the Keltic people known as the Dobunni, and both archaeology and genetics suggest that there's strong continuity between the two peoples, both demographically and culturally.
In other words, we would expect the tribe of Witches to be (culturally) a Kelto-Germanic amalgam.
Which, of course, is exactly what we are.
The eightfold Wheel of the Year as we know it today is a brilliant creation of 20th century Britain which, felicitously enough, would seem to be a reincarnation of the festal year of megalithic Britain.
One could call it Reinventing the Wheel.