Your code left the dataframe as it was and just replaced the day column with a sorted version.
Try
trip_weekday<- trip_weekday[trip_weekday$day, levels = c("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday","Saturday")],
I really like tidyverse where this would be:
trip_weekday <- trip_weekday %>% arrange(ordered(day, levels = c("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday","Saturday"))