Cron Expression
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A cron expression is a string of 5 or 6 fields that describes a task schedule. The fields specify minute, hour, day of month, month, and day of week. For example, 0 9 * * 1 means "every Monday at 9:00 AM".

Each field accepts specific values, ranges (1-5), lists (1,3,5), steps (*/10), or an asterisk (*) meaning "every value". The field order is: minute, hour, day of month, month, day of week.

The most common ones are: * * * * * (every minute), 0 * * * * (every hour), 0 0 * * * (daily at midnight), 0 9 * * 1 (Mondays at 9 AM), and 0 0 1 * * (first day of every month).

Cron is a classic Unix scheduler with a simple schedule syntax. Systemd timers are a newer Linux alternative that supports service dependencies, randomized delays, and logging through journald.