Originally Posted by George Steele's Barber
I sort of remember a backstage skit with Rusev, English, and New Day where Rusev Day was born. Maybe that was after the segment above. My memory isn't that clear on the matter.
To me Rusev Day is very clear. He is saying that everyday is for him and about him. Everyday is his day. Simple heel stuff driven by ego. It has caught on somewhat because vocal fans want more for Rusev than they feel that he is getting. There is also fear that he is going to leave if he isn't treated better and fans like him enough to stand up for him. It is also just kind of fun.
It's been chanted at non-WWE shows too. You're right it's just fun. People like what they like.