Cron expressions look like `0 9 * * 1-5`: at minute 0 of hour 9, every day, every month, on weekdays (Monday–Friday). In other words, 9am on weekdays.
The format dates back to 1970s Unix. Every modern scheduler. GitHub Actions, Vercel, Kubernetes CronJobs, AWS EventBridge, speaks it.
Special characters: `*` (every), `,` (list), `-` (range), `/` (step). Some systems add a sixth field for seconds or a year field, check your platform's docs.