I wrote a little bash script that displays the UF Calendar of the current month on your X11 root window (background). At the end of the month (starting at 28th), it also checks for the calendar of the next month and downloads it (once) automatically.

Just include it in your favourite window manager startup file. If you don't log out but use a screen saver or lock instead, you perhaps should make a cron job which calls that script at the first day of every month (and sets the DISPLAY variable).

#!/bin/bash
#
# Downloads and displays the UF Calendar, from Mirko Dölle <dg2fer@amsat.de>

# VERSION="NA"
VERSION="EU"

THISDAY=`date +"%e"`
THISMY=`LANG=C date +"%b%Y" | tr "[a-z]" "[A-Z]"`
NEXTMY=`LANG=C date -d "next month" +"%b%Y" | tr "[a-z]" "[A-Z]"`

if [ "${THISDAY}" -ge "28" ]; then
  if [ ! -e "UFCalendar_${NEXTMY}-${VERSION}.png" ]; then
    # try to download the calendar for the next month in background
    wget -o /dev/null -b -q \
  http://www.userfriendly.org/illiad/UFCalendar_${NEXTMY}-${VERSION}.png \
  >/dev/null 2>/dev/null
  fi
fi

if [ ! -e "UFCalendar_${THISMY}-${VERSION}.png" ]; then
  # we don't have the current calendar, so download it in foreground and
  # wait till download has finished
  wget -o /dev/null -q \
  http://www.userfriendly.org/illiad/UFCalendar_${THISMY}-${VERSION}.png \
  >/dev/null 2>/dev/null
fi

if [ -e "UFCalendar_${THISMY}-${VERSION}.png" ]; then
  # place here the command to display the calendar
  xv -root -quit UFCalendar_${THISMY}-${VERSION}.png
fi

You can get the script alternatively from http://www.mirko-doelle.de/uf-calendar.sh.