@RS One option would be this, assuming in this se you want to run the job at midnight on Dec. 25:
/bin/echo "/path/to/your/script.sh" | at midnight Dec 25
While cron doesn't offer a specific one-time execution, you could also do this in cron but would have to remove the entry afterwards:
0 0 25 12 * /path/to/your/script.sh
Also, take a look at this option: https://www.fastcron.com/guides/one-time-cronjobs/