"How do you celebrate Rosh Hashana? Ramadan? May day? Cinco de Mayo? Boxer day?"
It all depends. Normally, I do not celebrate these at all. However, if I am in company which does celebrate one or another of them, I am happy to participate in their celebration, just as I do at Christmas with my church-going family and friends. I am respectful of their beliefs, even though I do not share them. My atheism doesn't come up at all unless someone asks me directly.
When table grace is said, I lower my head and close my eyes, out of respect of the beliefs of others. I needn't believe as they do to show respect.
You are not participating, you are attending. Big difference.