Serenading usually happened on 1 of 2 occassions @ my school. The first was when you selected your chapters Lil' Bros/Sis and/or Sweetheart. The whole chapter goes and serenades them, gives them the fraternity's flower, and that's kind of how they find out about it
The other is @ parties (and formals, etc.) the fraternities often serenade their Sweetheart or Lil' Sis. The Phi Mu Alpha's were awesome @ this (it helps that 1/2 their chapter were vocal majors!).