Run a cron job on the first of every month: 0 0 1 * * guide

How this is calculated

Not every month has 31 days. If you specify the 31st, the job simply won't run in February, April, June, September, or November. For the last day of the month, cron alone can't express it. The common workaround is to run on the 28th-31st and have the script check if tomorrow is the 1st. Alternatively, use 0 0 28-31 * * and let the script exit early if the date isn't the last day. Systemd timers handle this natively with OnCalendar=*-*-28..31 00:00:00 plus a script guard.

Verdict

0 0 1 * * is correct for first-of-month execution. For last-day-of-month scheduling, use a wrapper script or switch to systemd timers. If you need the 15th, 20th, or any day that exists in every month, the standard day-of-month field works without caveats.