Apparently someone updated the wikipedia page on the Wheel of the Year to say that in 1970 Aidan A. Kelly came up with the idea of calling the Autumnal Equinox by the name of the Welsh hero Mabon ap Modron, dispite the lack of any conection between the hero and the equinox. And now smart asses are telling everyone who wishes people a "Happy Mabon" they are WRONG and totally "bogus".
Well, that argument hasn't stopped English speaking Christians from calling the resurrection of their god by the name of the Teutonic goddess of spring, Easter, even though it is called Passover in every other language.
Ár nDraíocht Féin has only been in existence since 1983 and call themselves Druids. They are hardly in a position to call other people "bogus".
Words mean what people use them to mean. As long as people know what you are talking about you are using words correctly.
So happy Mabon to everyone in the Northern Hemisphere and Happy Eostar to everyone in the Southern Hemisphere.