I remember when I first started to attend the SDA church people would say "Happy Sabbath" to me. And I thought... what strangeness is this? When I attended the Presbyterian church we didn't go around saying Happy Sunday. (Maybe some do but not any I've attended.) Adventists I think use it mostly in place of a "welcome, glad to see you" kind of thing. But really it means much more. It is a reminder (as is the Sabbath itself) that we should be HAPPY! We have a loving God who has set aside special time to spend with us!
I was always so happy as a child when my parents wanted to spend time with me. When they took off work or gave up something they wanted to do to be with me. How happy and special should we feel that the God of the universe will sit in our living rooms (He would probably not even mind sitting beside that lazy cat!), take a walk with us, read the Bible with us, visit a sick friend with us, etc. ESPECIALLY on the Sabbath. Thank you Lord for your love and watch-care over us even when we take it for granted.