Every festival on the wheel is older than the wheel. Samhain, Imbolc, Beltane and Lughnasadh are attested in early Irish sources as the hinges of the agricultural year; midwinter and midsummer observances are widespread in the European record; and the equinoxes have been marked wherever astronomy was practised. But the eightfold scheme that joins them, every festival kept by the same people in a single turning cycle, is a modern construction, and the scholarship on modern Paganism has traced its assembly in detail.
The historian Ronald Hutton locates the join in the mid twentieth century, when Gerald Gardner's early Wiccan coven, which kept the four cross quarter nights, and the modern Druid orders, which kept solstices and equinoxes, brought their calendars together; by the end of the 1950s the eight festival year was standard in Wicca and spread outward from it.
Some of the names are newer still: Litha, Mabon and Ostara were applied to the solstice and equinox festivals in the 1970s, borrowings from Bede and from Welsh legend rather than survivals. None of this diminishes the wheel, and the traditions that keep it increasingly say so themselves: it is a modern liturgical calendar built from genuinely old parts, and its honesty about that construction is part of the story the atlas records.