Great question! In my sentence I should use "your" to show that something belongs to someone. So, if I say, "I'm sorry to interrupt your weekend," I'm saying that I'm interrupting the weekend that belongs to you. But if when I wrote, "I'm sorry to interrupt you weekend," [and I often do without noticing it] it's confusing because it doesn’t show that "weekend" is yours. It sounds as if I'm trying to "interrupt" you directly [and sorry if I did], rather than interrupting your weekend. Using "your" makes it clear that I'm sorry to interrupt the time that belongs to you – your weekend.

In regard to "Hello all."  This is a greeting directed to a group of people, addressing "all" as a collective noun. It's an informal way of saying "Hello, everyone."