Great day in the morning is just a saying that basically means either the same as "my goodness' or "my gracious" or oh hell.
It can mean that something bad is about to happen such as on Sanford and Son where Fred Sanford opens the door and a person supposedly from the New York Times wants to take his picture and interview him about his winning money.
But then the guy pulls out a gun and robs him and then he says great day in the morning which is meaning oh my god or oh my goodness or oh hell I'm about to be robbed.
That's essentially what it means.