Firs time user - Find an average total for Monday orders?

[NancyNewbie]

Hello,
As my name shows, I am a newbie. This is my first time even asking a question. Bear with me!

I am working on a data set that has orders and dates associated with the orders. I need to find the average total for Monday orders, but none of the data sets list the days. Feeling pretty stupid, but yes, do not even know how to begin. Help?

[FJCC]

Here is an example of one way to add a column that shows the day of the week.

library(lubridate)
DF <- data.frame(DATE=seq.Date(from = as.Date("2020-11-01"),
                                to=as.Date("2020-11-23"),by="day"),
                  Value=1:23)
DF$DayOfWeek <- wday(DF$DATE,label = TRUE)
head(DF)
        DATE Value DayOfWeek
1 2020-11-01     1       Sun
2 2020-11-02     2       Mon
3 2020-11-03     3       Tue
4 2020-11-04     4       Wed
5 2020-11-05     5       Thu
6 2020-11-06     6       Fri