This is a tricky one. Use the following cron:
0 0 1-30/3 * * commandThe day-of-month range 1-30/3 expands to 1, 4, 7, 10, 13, 16, 19, 22, 25, 28 — ten runs per month, every 3 days starting from the 1st. Why 1-30 and not 1-31? Because we want consistent “every 3rd day” behaviour across months; if you include 31, the cadence resets weirdly between months that have 31 days and months that don't.
Caveat: this resets every month boundary. Going into February, the run lands on the 1st (because the cycle starts there), even if the previous month's last run was on the 28th — only 4 days earlier instead of 3. For a strict every-72-hours cadence that doesn't reset mid-month, cron is the wrong tool — use a job runner that supports interval-based scheduling (Kubernetes CronJob with custom logic, or a periodic timer in your runtime).
“Every 3 days” can also mean “Mon-Wed-Fri” for weekdays — for that pattern, see every Monday, Wednesday and Friday.
Read the dedicated guide: Cron every day.
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