Construction of a random date generator with weighted weekends in R - r

Question

I am performing creel surveys and am attempting to construct a random date generator that weights the weekends higher than the weekdays. So far I have a simplistic random date generator that does not take into account the day type. We expect more angling pressure on the weekends (as that is when more people have time to fish) but do not have a way to select random days without including bias. I would like to select 15 days within a given month.
I've already generated a simplistic random date generator:
dates <- data.frame(seq.Date(as.Date(day.start),as.Date(day.end),by="day"))
dates
sample(dates$seq.Date.as.Date.day.start...as.Date.day.end...by....day.., size = 15, replace = FALSE)
[1] "2019-11-10" "2019-11-06" "2019-11-04" "2019-11-27" "2019-11-30" "2019-11-15"
[7] "2019-11-18" "2019-11-21" "2019-11-13" "2019-11-01" "2019-11-19" "2019-11-25"
[13] "2019-11-07" "2019-11-02" "2019-11-23"
Ideally I would have an end product that allows me to input the month start and end and outputs 15 random days.

Answer 1

Explanation in comments in code below:
# Generate initial data; as in question
day_start <- as.Date("2010-10-01")
day_end <- as.Date("2010-10-31")
dates <- data.frame(date = seq.Date(day_start, day_end,by="day"))
# Determine inclusion probabilities for each date; give weekend a higher
# probability.
dates$day <- as.numeric(format(dates$date, "%u"))
dates$psamp <- ifelse(dates$day >= 6, 0.2, 0.1)
# Make sure probabilites add up to requires sample size
samplesize <- 15
dates$psamp <- dates$psamp * samplesize/sum(dates$psamp)
# Do not use sample for sampling without replacement with unequal probabilities!
# The sampling package has a large number of routines for sampling without
# replacement and unequal probabilites. The following gives a fixed size sample
# (sum dates$psamp)
library(sampling)
dates$selected <- UPrandomsystematic(dates$psamp)
As for the reason why I don't use sample see, for example, https://stat.ethz.ch/pipermail/r-help/2008-February/153601.html.

Answer 2

Here's a somewhat general function that does what you want. It takes the start day, end day, and the weight (relative to 1) that you want to put on weekends as its own arguments, and passes on other additional arguments (size, replace, etc.) to sample. No dependencies other than base R.
However, if sampling without replacement, you may want to use the sampling package as recommended in Jan van der Laan's answer.
rday = function(
start_day = as.Date("2019-01-01"),
end_day = as.Date("2019-01-31"),
weekend_weight = 2,
...
) {
if (! "Date" %in% class(start_day)) start_day = as.Date(start_day)
if (! "Date" %in% class(end_day)) end_day = as.Date(end_day)
dates = seq(start_day, end_day, by = "1 day")
weights = rep(1, length(dates))
weights[weekdays(dates) %in% c("Saturday", "Sunday")] = 1
sample(dates, ..., prob = weights)
}
rday(size = 15)
# [1] "2019-01-24" "2019-01-07" "2019-01-21" "2019-01-15" "2019-01-27" "2019-01-04" "2019-01-30" "2019-01-12"
# [9] "2019-01-11" "2019-01-08" "2019-01-20" "2019-01-01" "2019-01-03" "2019-01-19" "2019-01-29"