Well, without a reproducible example, I couldn't come up with a complete solution, but here is a way to generate the first Wednesday date of each month. In this example, I start at 1 JAN 2013 and go out 36 months, but you can figure out what's appropriate for you. Then, you can check against the first Wednesday vector produced here to see if your dates are members of the first Wednesday of the month group and assign a 1, if so.
# I chose this as an origin
orig <- "2013-01-01"
# generate vector of 1st date of the month for 36 months
d <- seq(as.Date(orig), length=36, by="1 month")
# Use that to make a list of the first 7 dates of each month
d <- lapply(d, function(x) as.Date(seq(1:7),origin=x)-1)
# Look through the list for Wednesdays only,
# and concatenate them into a vector
do.call('c', lapply(d, function(x) x[strftime(x,"%A")=="Wednesday"]))
Output:
[1] "2013-01-02" "2013-02-06" "2013-03-06" "2013-04-03" "2013-05-01" "2013-06-05" "2013-07-03"
[8] "2013-08-07" "2013-09-04" "2013-10-02" "2013-11-06" "2013-12-04" "2014-01-01" "2014-02-05"
[15] "2014-03-05" "2014-04-02" "2014-05-07" "2014-06-04" "2014-07-02" "2014-08-06" "2014-09-03"
[22] "2014-10-01" "2014-11-05" "2014-12-03" "2015-01-07" "2015-02-04" "2015-03-04" "2015-04-01"
[29] "2015-05-06" "2015-06-03" "2015-07-01" "2015-08-05" "2015-09-02" "2015-10-07" "2015-11-04"
[36] "2015-12-02"
Note: I adapted this code from answers found here and here.