The Sunday poll would be most valid because everyone’s home watching their local NFL team. Friday and Saturday not so much and so they have to fudge the numbers to try to create a valid sample. That’s when the unbalanced D versus R sampling takes place.
So of the three days polled, Sunday would be the most accurate.
There's a counter-argument that Sunday would be biased in favor of Democrats because Republican-leaning voters are in church all morning.