preperation

First, install xsnow to your system. If your system is based on RPMs find the right rpm, are for dependencies and .... good luck. If you use the debian distribution just get it from the network

apt-get install xsnow

configuration

Find the directory with KDMs config files. In debian, there are in /etc/kde3/kdm/. You have to edit two files: One for starting xsnow with the greeter, and one to stop zsnow, as soon somebody logs in.
From my /etc/kde3/kdm/Xsetup. The commands below start xsnow in Nov. and Dec. only:

if [ `date +%m` -ge 11 ]; then
   exec /usr/games/xsnow  &
   echo $! > /var/run/xsnow.pid
fi

From my /etc/kde3/kdm/Xstartup:

killall xsnow
# kill `cat /var/run/xsnow.pid`

advancement

Some prefer more fancy settings to define the dates when xsnow should start. Check the mail archiv for details

,----[ Xsetup ]
| #! /bin/sh
| # Xsetup - run as root before the login dialog appears
| 
| if [ `date +%m` -ge 11 ] || [ `date +%m` -le 2 ]
| then
|     if [ `date +%m` -eq 12 ] && [ `date +%m` -ge 26 ]
|     then
|         exec /usr/bin/xsnow   &
|         echo $! > /var/run/xsnow.pid
|     else
|         /usr/bin/xsnow -notrees -nosanta &
|         echo $! > /var/run/xsnow.pid
|     fi
| fi
| 
`----