Lammas < Old English hlæfmæsse, “loaf mass.”
“A harvest festival formerly held in England on August 1 when bread baked from the season's first ripe grain was consecrated.”
Blessing the harvest's first grain: something we've probably been doing since the end of the last Ice Age.
But (etymologically speaking, anyway) the name is inescapably Christian. What to do?
Well, there's always Lúnasa. (That's the simplified Modern Irish form of the feast known in Old Irish as Lughnasadh [and by many other spellings]).
But that name has problems of its own. For one, it's specific to a specific culture and a specific pantheon. For another, for English-speakers, it is and always will be a foreign import.
Some Old Craft folks that I know wouldn't be caught dead using a Pagan Revival term like Lúnasa. In the old days, under the radar was the only safe way to fly. Where they come from, it's Lammas all the way.
Here's one you probably haven't heard before: the Gule of August. We get Gule (rhymes with Yule, which is nice) from French, although there's a Welsh form (Gwyll) too; its ultimate origin may be Latin vigilia, “vigil.” Well, in the Wonderful World of Polytheism more is generally better; “Gule” is fine if you want to mystify your friends.