Cron Expression to Run Every Third Day
The common answer is 0 0 */3 * *, but the 3-day count restarts every month, so the cadence is not a clean 72 hours year-round. The explanation and a fix are below.
The expression
0 0 */3 * * runs at midnight on day 1, 4, 7, 10, … of each month — roughly every third day.
Why it drifts across months
The */3 step counts from day 1 within the day-of-month field. At the start of a new month it resets to day 1, so the gap between the last run of one month and the first of the next is usually not exactly three days. A 31-day month runs on the 31st, then the 1st — a one-day gap.
If you need an exact 72-hour cadence
Plain cron cannot track a true every-72-hours schedule across month boundaries. Use a systemd timer with OnUnitActiveSec=3d, or have the script record the last run date and exit early if fewer than three days have passed (run it 0 0 * * * daily and let it self-gate).
FAQ
What is the cron for every 3 days?
0 0 */3 * * runs at midnight on day 1, 4, 7, 10 ... of each month.
Why isn't every-3-days exact?
The day-of-month step resets on the 1st, so month boundaries break the 3-day rhythm (e.g. a run on the 31st then the 1st).
How do I run exactly every 72 hours?
Use a systemd timer with OnUnitActiveSec=3d, or run daily and have the script skip unless 3 days have passed since the last run.