How to read a cron schedule
Cron is the standard way to schedule recurring tasks on Unix-like systems, and a cron expression is five fields separated by spaces: minute, hour, day-of-month, month, and day-of-week. Each field holds a number, a list, a range, or an asterisk meaning "every". So 0 9 * * 1 means minute 0, hour 9, every day-of-month, every month, on Monday, in other words 9:00 every Monday.
The building blocks. An asterisk * means every value; */5 in the minute field means every five minutes; a comma lists values (1,15); a hyphen gives a range (9-17 for business hours). Days of the week run 0–6 from Sunday, and many systems accept names like MON. The tool above turns any expression into plain English so you can confirm it does what you meant.
The traps. The two day fields (day-of-month and day-of-week) are OR-combined when both are set, which surprises people. Cron also uses the server's timezone, so a job may run at an unexpected local time, and it does not catch up on runs missed while the machine was off. Always sanity-check a new schedule against a human-readable translation before relying on it.