My lucky daughter is spending a week in Maui. I sent her a text and started to write, "Let me know when you arrive." However, I changed it to "Let me know when lucky you arrives."

In the latter "arrives" seemed right, versus "arrive" in the former sentence. Why is that?

lucky you is third person. It's a mention of you instead of a use?

this is what I saw in Maui

That's what Shu thought. Thanks, goofy!