You need to make your Day column a factor with the levels set in the order that you want them on the x axis. Here is an example with just four days.
library(ggplot2)
DF <- data.frame(Day = c("Friday", "Monday", "Sunday", "Saturday"),
n = c(23,18,28,21))
DF$Day <- factor(DF$Day, levels = c("Monday", "Friday", "Saturday", "Sunday"))
ggplot(DF, aes(Day, n)) + geom_col(fill = "steelblue")