Run a cron job every three days

You can use a day-of-month step when you want a repeating date pattern, but standard cron cannot express a continuous 72-hour interval across month boundaries. The expression below runs at midnight on dates 1, 4, 7, 10, 13, 16, 19, 22, 25, and 28 of every month.

# at midnight on dates 1, 4, 7, and so on through 28
0 0 1-30/3 * *

Read 0 0 1-30/3 * * from left to right: the first 0 selects minute 0; the second 0 selects hour 0; 1-30/3 starts at day 1 of the month and advances through that range in steps of three; the month * allows every month; and the day-of-week * allows every day of the week. This is suitable when those calendar dates are what you mean. It is not a strict “wait three days after the previous run” schedule because the day-of-month field restarts at 1 each month. If elapsed time matters, run a daily cron and let your command compare a persisted last-run timestamp before doing the work:

# daily trigger; the script exits unless 72 hours have passed
0 0 * * *