There's really no clear evidence or proof that pumping gasoline in the morning gets you more gasoline and more energy from the gasoline than it does when pumped at night.
According to some people the gas you pump in the morning is denser in the cool early morning than it is in the daytime or night which supposedly means you get more energy from the gasoline.
However there's no evidence to that and it's likely untrue so it really does not matter whether you pump the gasoline in the daytime or night time as it really won't make a difference.
Also at night it usually cools down as well so the gasoline should be denser then as well.
I never worry about when I pump my gas and just pump the gasoline when needed.
However I do not pump gas when the gasoline station is having their tanks filled due to the potential of them stirring up debris in the tanks.