Вот простой способ сделать это:
mydata<-data.frame(NO_Empl=c(127,125,124),Month=c(9,8,7),Year=c(2017,2018,2017))
library(lubridate)
library(tidyverse)
as.tibble(mydata) %>%
mutate(Date=make_date(Year,Month)) %>%
select(-Month,-Year) %>%
mutate(Day=wday(Date),Day_date=month(Date))
Результат:
# A tibble: 3 x 4
NO_Empl Date Day Day_date
<dbl> <date> <dbl> <dbl>
1 127 2017-09-01 6 9
2 125 2018-08-01 4 8
3 124 2017-07-01 7 7