I'm always able to get a FastPass for FoP the day of, usually they appear around 10am. I have to refresh the app (about 38 thousand times.) But that's for a party of one keep in mind.
I do that all the time btw. I got a party of two on Toy Story Midway a couple weeks ago on a Saturday night for just 1 hour away. Just gotta keep refreshing. Over and over and over and over and over.
The trick is to pick a time for something else, swipe the times bar right or left to force it to load more times for that attraction, then select the back button, if the one you want is still not available, pick any other attraction, and repeat, forever, until one appears.
I'm so good at it I do it in my sleep now.