:-) I believe (but don't bet too much on it) that Lammas is essentially the Christian evolution of Lughnasadh. Lughnasadh was certainly a celebration of one of Ireland's great heroes, Lugh-of-the-long-hand, from its ancient mythological cycle. It was very common to replace established festivals, gods, heroes, etc when bringing in the new Order of Things. In other words, you don't have to change the celebration, just its name.

I don't know any really good term for Neo-pagan/Acquarian/New Age/Wiccan practioners (and maybe it's inappropriate to lump them together at all) but "they" are celebrating a seasonal cycle event. "They're" entitled to call it or practice at it whatever they wish. I would bet (but not the farm) that there is no continuity anywhere in Northern Europe from any Old Religion to any New. Thus, there can be no such thing as Orthodoxy in this matter.