The cron expression for first of the month

To run a job first of the month, use 0 0 1 * *. Read aloud it means: At 00:00 on day-of-month 1.

Field by field: minute 0 — minute 0; hour 0 — hour 0; day of month 1 — day-of-month 1; month * — every month; day of week * — every day-of-week.

Drop the expression into a crontab, a CI schedule, or anything else that speaks standard five-field cron syntax. The times above are in your local timezone — most cron daemons run in the server's timezone, so mind the difference when it matters.