If you work a Mon-Fri job, could it be that you’re just the most rested on Sunday mornings?
I drink 1-2 beers most nights, maybe 3 on when out to dinner on Saturdays, and my Sunday runs used to always be the longest and the most fun even if I picked very technical trails and pushed hard. Now that I work largely from home, I schedule my long runs around deadlines, and I run best when I’ve had a day to breathe, regardless of alcohol intake (if any) the night before.