Just say "Eid Mubarak" with a warm smile, should be good. Could be followed by a friendly hug and a gift of sweetmeats but that's mostly for family & friends.
And for the curious: the proper response to an "Eid Mubarak!" is "Khair Mubarak!"
"Eid" is the day, and "Mubarak" means merry/blessed. So "Happy Eid" could be the sentiment if you're uncomfortable with pronouncing "Mubarak" (which is completely phonetic, by the way) or just "Eid Mubarak."
You can't post slurs or religious flamewar to HN. As you've done so repeatedly in this thread, I've banned this account.
If you don't want to be banned, you're welcome to email hn@ycombinator.com and give us reason to believe that you'll follow the rules in the future. They're here: https://news.ycombinator.com/newsguidelines.html.
I barely know them they’re new, but want them to feel welcome, think they might be the only Muslim neighbors