The event name point is well taken... This would be "our" event to plan, organize, and participate in (hence the "Let's" in the thread title). It's out here in the open now and in no way exclusionary. Of course we should build on what's been done in the past (as I wrote earlier). So, in the spirit of politeness, this is a formal "reaching out"--"Let's" do this
I agree with previous comments that "Indiana Legal Open Carry Day" could be perceived as implying that that it is legal only on that day etc by the less educated, or/and observant, in our State.
May I suggest a slight change?
"Indiana 'Open Carry is Legal' Day"
..or something similar?
I would also think it would be nice for you to reach out to the others on here that have done this before, and at least offer to work with them, rather than making this a separate event. It would be the polite thing to do, I think...