Format
┌─ minute (0–59)
│ ┌─ hour (0–23)
│ │ ┌─ day of month (1–31)
│ │ │ ┌─ month (1–12 or JAN–DEC)
│ │ │ │ ┌─ day of week (0–6 or SUN–SAT; 7 is also Sunday)
* * * * *
Each field takes *, a value, a list 1,15, a range 9-17, or a step */15 / 0-30/10. Macros @hourly, @daily, @weekly, @monthly and @yearly are expanded.
The day-of-month / day-of-week trap
When both day fields are restricted, standard (Vixie) cron runs when either matches: 0 0 13 * 5 runs on every 13th and every Friday, not only on Friday the 13th. The next-run list makes this visible.
The plain-English description comes from cronstrue; next runs are computed here, minute by minute, in your local time zone or UTC.