FreeBSD: Install from /usr/ports/sysutils/bsdstats
OpenBSD: OpenBSD does not have the 'periodic' system, but instead has the /etc/daily, /etc/weekly and /etc/monthly scripts, all of which are executed at the corresponding intervals from root's crontab[1].
The daily, weekly and monthly scripts will run the contents of corresponding .local files, so to run the bsdstats script once a month you either
create an /etc/monthly.local script which calls the 300.statistics script,
or, if the the 300.statistics script is the only local script you want to run each month, copy 300.statistics to /etc/monthly.local.
The bsdstats script depends on the setting of some environment variables which on FreeBSD are kept in /etc/periodic.conf. On OpenBSD, the place for locally defined system variables is /etc/rc.conf.local, where you put the lines
monthly_statistics_enable="YES" # needed for bsdstats to work at all
As indicated by the comments, only the first of those is mandatory for the script to work; the second lets you submit hardware information too if you want to.
With a valid /etc/monthly.local script which runs the statistics script and the /etc/rc.conf.local lines in place, you can either sit back and wait for the monthly script to execute at some point in the future or run the script manually.
[1] sudo crontab -u root -e will show the times and other data about how the scripts are run.
Other BSDs:
The following variables can be set in /etc/rc.conf.local to enable the statistics, as well as fine tune various parts of it:
After downloading, run it once initially.