If I understand correctly you want the number of days per site. If your data looks like this:
site = c("a", "b", "c", "a", "b", "c", "a", "b", "c")
year = c(1991, 1992, 1993, 1991, 1992, 1993, 1991, 1992, 1993)
month = c(1, 1, 4, 4, 1, 1, 4, 4, 1)
my_data = data.frame(site, year, month)
You can use the package dplyr (install via install.packages(dplyr)):
library(dplyr)
my_data %>% group_by(site) %>% count(year, month)
Output:
# A tibble: 6 x 4
# Groups:   site [3]
  site   year month     n
  <chr> <dbl> <dbl> <int>
1 a      1991     1     1
2 a      1991     4     2
3 b      1992     1     2
4 b      1992     4     1
5 c      1993     1     2
6 c      1993     4     1
You can post a snippet of your code using dput:
dput(my_data)
structure(list(site = c("a", "b", "c", "a", "b", "c", "a", "b", 
"c"), year = c(1991, 1992, 1993, 1991, 1992, 1993, 1991, 1992, 
1993), month = c(1, 1, 4, 4, 1, 1, 4, 4, 1)), class = "data.frame", row.names = c(NA, 
-9L))
Then other people can take that the above code and reproduce your data. It doesn't have to be ALL your data, just the first ~10 rows.