The /etc/monthly shell script is run once per month to handle basic system administration. By default, it does almost nothing except run /etc/ monthly.local. This script does contain a command to provide monthly user login time accounting, but it is commented out. To activate it, just uncomment the lines around "login accounting." This was very important back when computer time was a valuable resource that needed to be accounted for, but is less so now. It can be useful to see which users access the system most frequently, however.