Finding the last day of 3rd week for each month in R

I have a vector that contains dates. I want to find the dates which are the last day of the 3rd week for each month.

structure(c(18631L, 18632L, 18633L, 18634L, 18635L, 18638L, 18639L, 18640L, 18641L, 18642L, 18645L, 18646L, 18647L, 18648L, 18649L, 18651L, 18652L, 18653L, 18654L, 18655L, 18656L, 18659L, 18660L, 18661L, 18662L, 18663L, 18666L, 18667L, 18668L, 18669L, 18670L, 18673L, 18674L, 18675L, 18676L, 18677L, 18680L, 18681L, 18682L, 18683L, 18684L, 18687L, 18688L, 18689L, 18690L, 18691L, 18694L, 18695L, 18696L, 18697L, 18698L, 18701L, 18702L, 18703L, 18704L, 18705L, 18708L, 18709L, 18710L, 18711L, 18712L, 18715L, 18716L, 18717L, 18718L, 18722L, 18723L, 18724L, 18725L, 18726L, 18729L, 18730L, 18731L, 18732L, 18733L, 18736L, 18737L, 18738L, 18739L, 18740L, 18743L, 18744L, 18745L, 18746L, 18747L, 18750L, 18751L, 18752L, 18753L, 18754L, 18757L, 18758L, 18759L, 18760L, 18761L, 18764L, 18765L, 18766L, 18767L, 18768L), class = c("IDate", "Date" ))

I have tried to solve this using lubridate but could not succeed. I am sure someone should be able to figure it out quickly.

Solution

This depends on how you define weeks in each month as stated by @IRTFM. However, here is one attempt at this using base R taking help from the function here if d is the vector of dates.

monthweeks <- function(x) {
  ceiling(as.numeric(format(x, "%d")) / 7)
}

d[monthweeks(d) == 3 & weekdays(d) == 'Friday']
#[1] "2021-01-15" "2021-02-19" "2021-03-19" "2021-04-16" "2021-05-21"