Day of the week number in R

There is R base function like weekdays that can return the name of the weekday. But what if you want to get the day of the week number in R to do calculations? Here is how to do that by using one of the most popular R packages.

If you want to test the weekdays function, then here it is. That will return the full name of the weekday as it is defined in the regional settings.

weekdays(Sys.Date())

Day of the week number in R

If you want to get the day of the week number in R then one of the best options is the wday function from data.table or lubridate package. In both of them, functions do the same, but I would like to recommend wday from lubridate. Just because there are some handy options, like the week_start option. Of course, you can also do the necessary adjustments mathematically.

lubridate::wday(Sys.Date(), week_start = 1)
#[1] 1

Extract the date of first Monday from a list of dates in R

If you want to select data from the first occurring Monday to group by week numbers or do other things, then it is good to know how to get a number of the weekday.

Here is a data frame that contains dates for the last 30 days.

x <- data.frame('date' = seq(Sys.Date()-30, Sys.Date(), by="days"))

head(x)
#        date
#1 2021-01-23
#2 2021-01-24
#3 2021-01-25
#4 2021-01-26
#5 2021-01-27
#6 2021-01-28

What is the date of the first Monday in the previous 30 days? You can do it in many ways in here is one.

require(dplyr)

x %>%
  mutate(wd = lubridate::wday(date, week_start = 1)) %>%
  filter(wd == 1) %>%
  distinct(wd, .keep_all = T)

#        date wd
#1 2021-01-25  1

I hope it was helpful for you. Especially if you have Excel knowledge, then it looks similar to what you can get with a function called WEEKDAY.

If you are transitioning from Excel to R then this post might be helpful for you.

If not, then a few tips and tricks in RStudio are always useful.