R find last weekday of month - r

Question

How do I find the last weekday (e.g., Wednesday) of the month using R? In the code below, I calculate the month, day of the month, week of the month, and weekday. There are 5 Wednesdays in January 2014, but only 4 Wednesdays in February 2014, so I cannot use max(week of the month) as a filter. Any help is appreciated although I prefer to use the base R functions.
DF <- data.frame(DATE = seq(as.Date("2014-01-01"), as.Date("2014-12-31"), "day"))
DF$MONTH <- as.numeric(format(DF$DATE, "%m"))
DF$DAY_OF_MONTH <- as.numeric(format(DF$DATE, "%d"))
DF$WEEK_OF_MONTH <- ceiling(as.numeric(format(DF$DATE, "%d")) / 7)
DF$WEEKDAY <- format(DF$DATE, "%A")
DF

Answer 1

I think this is what you're after:
DF$last_weekday_o_month <- ave(
weekdays(DF$DATE),
months(DF$DATE),
FUN = function(x) tail(x[ !(x %in% c("Saturday","Sunday")) ], 1)
)
To find the particular date that is the last weekday....
DF$last_weekdaydate_o_month <- ave(
DF$DATE,
months(DF$DATE),
FUN = function(x) tail(x[ !(weekdays(x) %in% c("Saturday","Sunday")) ], 1)
)
the result looks like...
DATE last_weekday_o_month last_weekdaydate_o_month
1 2014-01-01 Friday 2014-01-31
2 2014-01-02 Friday 2014-01-31
3 2014-01-03 Friday 2014-01-31
4 2014-01-04 Friday 2014-01-31
5 2014-01-05 Friday 2014-01-31
6 2014-01-06 Friday 2014-01-31
...
360 2014-12-26 Wednesday 2014-12-31
361 2014-12-27 Wednesday 2014-12-31
362 2014-12-28 Wednesday 2014-12-31
363 2014-12-29 Wednesday 2014-12-31
364 2014-12-30 Wednesday 2014-12-31
365 2014-12-31 Wednesday 2014-12-31
If you did this first, of course you could compute last_weekday_o_month as weekdays(last_weekdaydate_o_month).
With a couple packages, this can be done more elegantly/readably, as suggested by #RichardScriven:
library(data.table)
setDT(DF)[,
last_weekdaydate_o_month := last(DATE[!chron::is.weekend(DATE)])
, by = month(DATE)]
which gives
DATE last_weekdaydate_o_month
1: 2014-01-01 2014-01-31
2: 2014-01-02 2014-01-31
3: 2014-01-03 2014-01-31
4: 2014-01-04 2014-01-31
5: 2014-01-05 2014-01-31
---
361: 2014-12-27 2014-12-31
362: 2014-12-28 2014-12-31
363: 2014-12-29 2014-12-31
364: 2014-12-30 2014-12-31
365: 2014-12-31 2014-12-31

Answer 2

Here is a method using dplyr. Essentially you group by the month, filter out the 'weekend' days and return the weekday of the last (i.e. max) day.
library(dplyr)
DF <- data.frame(DATE = seq(as.Date("2014-01-01"), as.Date("2014-12-31"), "day"))
DF %>%
mutate(month = months(DATE), weekday = weekdays(DATE)) %>%
group_by(month) %>%
filter(!weekday %in% c("Saturday", "Sunday")) %>%
summarise(last_weekday = weekdays(max(DATE)))
Source: local data frame [12 x 2]
month last_weekday
1 April Wednesday
2 August Friday
3 December Wednesday
4 February Friday
5 January Friday
6 July Thursday
7 June Monday
8 March Monday
9 May Friday
10 November Friday
11 October Friday
12 September Tuesday

Answer 3

library(lubridate)
x <- seq(as.Date("2007-12-31"), by="1 day", length.out=(Sys.Date() - as.Date("2007-12-31")))
library(plyr)
df <- data.frame(date=x, year=year(x), month=month(x))
df[,"weekday"] <- weekdays(df[,"date"])
df<- df[! df[,"weekday"] %in% c("Saturday", "Sunday"),]
df <- ddply(df, .(year, month), summarize, last=max(date))