I love the general questions, @daemonfowl, because they are of interest to a wide audience.
As to this question, it's easy enough to set specific times of day, or day of the week, or day of the month, or any recurring pattern you like for any executable task to run. The term used on Unix-like systems is "cron job". Start with
cron(8), and then follow the SEE ALSO section for the management commands and file format you will need to learn.
When you first installed OpenBSD, there was mail in root's inbox suggesting you read the
afterboot(8) man page. In that, you will find a discussion of OpenBSD's daily, weekly, and monthly management scripts, and you can review root's crontab file to see when those are run, as examples of setting up recurring scripts.