The RcppBDT package wraps calculations with the date (and associated time) from Increase table_time .
Among them, there is a function that does what you want here:
R> getNthDayOfWeek(third, Fri, Apr, 2014)
[1] "2014-04-18"
R>
Instead of the (local-local) constants Jan, Feb, ... you can also use integers to fill you in three to six months.
Here is a simple (ugly) hack for the sequence:
R> dates <- data.frame(mon=c(5:12, 1:12, 1:12, 1:4),
+ year=c(rep(2011,8), rep(2012,12),
+ rep(2013,12), rep(2014,4)))
R> sapply(1:36, function(i)
+ format(getNthDayOfWeek(third, Fri, dates[i,1], dates[i,2])))
[1] "2011-05-20" "2011-06-17" "2011-07-15" "2011-08-19"
[5] "2011-09-16" "2011-10-21" "2011-11-18" "2011-12-16"
[9] "2012-01-20" "2012-02-17" "2012-03-16" "2012-04-20"
[13] "2012-05-18" "2012-06-15" "2012-07-20" "2012-08-17"
[17] "2012-09-21" "2012-10-19" "2012-11-16" "2012-12-21"
[21] "2013-01-18" "2013-02-15" "2013-03-15" "2013-04-19"
[25] "2013-05-17" "2013-06-21" "2013-07-19" "2013-08-16"
[29] "2013-09-20" "2013-10-18" "2013-11-15" "2013-12-20"
[33] "2014-01-17" "2014-02-21" "2014-03-21" "2014-04-18"
R>
, format(), ; . sapply(), , , Date dates, .