awk recipe which takes the output of `cal` command and colorizes it. Useful for geektool.

gistfile1.awk

##
## only warks with GNU awk (gawk)
## highlighting for today's date can be disabled with skiptodayhighlight=1
## command-line parameter
##
#
#
# For color codes see:
#       http://www.termsys.demon.co.uk/vtansi.htm
# print input string in yellow
function yellow(string) {
    # \033[0m is used to reset escape sequence
    printf("%s%s%s", "\033[1;33m", string, "\033[0m");
}
# print input string in cyan
function cyan(string) {
    # \033[0m is used to reset escape sequence
    printf("%s%s%s", "\033[1;36m", string, "\033[0m");
}
# print input string in red
function red(string) {
    # \033[0m is used to reset escape sequence
    printf("%s%s%s", "\033[1;31m", string, "\033[0m");
}
# print input string in green
function green(string) {
    # \033[0m is used to reset escape sequence
    printf("%s%s%s", "\033[1;32m", string, "\033[0m");
}

# since this will be used with output from cal, assume fixed width fields
# of 3 characters
BEGIN   {
    FIELDWIDTHS = "3 3 3 3 3 3 3"
    today = strftime("%e")
}
# print month in yellow
NR==1   {print yellow($0)}
# print the weekdays in green
NR==2   {print green($0)}
# for rows greater than 2
NR>2  { 
    # iterate over all the columns
    for (i=1; i<=NF; i++) {
        # use first 2 characters only
        date = substr($i, 1, 2)
        if (i == 1 || i==NF) { # first column or last column
            # print in red if today's date and not skipping, otherwise cyan
            date = today == date && !skiptodayhighlight ? red(date) : cyan(date)
            # print newline if last column
            printf("%s%s", date, i == NF ? ORS : OFS)
        } else { # middle columns
            # print in red if today's date and not skipping, otherwise normal
            date = today == date && !skiptodayhighlight ? red(date) : date
            printf("%s%s", date, OFS)
        }
    }
}

Sample output where today's date is highlighted in red, weekends are in cyan.

Use as follows:

cal | gawk -f ~/etc/calendar.awk