Answer by Kamil Maciorowski for Setting up a cron job which runs on the Monday of the week which contains the 3rd Thursday

The first Thursday of a month is between the 1st and the 7th day of the month (inclusive). The second Thursday is between 8th and 14th, the third Thursday is between 15th and 21st.

The Monday in the week of the third Thursday is exactly three days before the third Thursday, so between 12th and 18th day (inclusive).

You need your cron job to run on Monday between 12th and 18th day of each month.

A job with initial fields 30 8 12-18 * MON is not right for this because it would run each day from 12th to 18th and on every Monday outside of the range. This is a known quirk of cron. Answers to the linked question provide methods to deal with the quirk. There are methods that require some logic in the script cron is going to run; and there are methods that put all the logic inside crontab. Choose whatever fits your needs.

E.g. the method from this answer tailored to your case will look like this in your crontab:

30 8 */100,12-18 * MON /path/to/executable

Notes:

• There are various implementations of cron. It may be some methods don't work with some implementations.

• Some implementations of cron provide a system-wide crontab that requires an extra field. See this question. If you choose to use the system-wide crontab then build your crontab entry accordingly.