>> Maybe it was a convenient post-harvest period for work, or an intra-harvest exercise

I think this was basically the theory about Newgrange, but I think that Newgrange's stages were during winter.

I'm not saying that there is a Lughnasadh tie-in because of the August link, just saying that the evidence pointss to a strong August link.

I must say though that a successfully gathered harvest is a time for celebration and always has been. A poor harvest is a time to say sorry to the gods for whatever it is that you or your king did to piss them off. It's not necessarily time to go and build a bloody great hill for the fun of it :-)