Crontab - Run a cronjob quarterly

The simplest solution here would be to have two entries in your crontab--one for the 30s and another for the 31s, e.g.:

 0 0 30 6,9 * /path/to/your/script
 0 0 31 3,12 * /path/to/your/script

At 00:00 on day-of-month 1 in every 3rd month

0 0 1 */3 *

from https://crontab.guru/every-quarter