A 21-on/21-off work rotation repeats every 42 days. Treat your first workday as day one. The first work block ends 20 days after that date, and your first day off starts 21 days after it.
For example, if you start on January 4, 2027, the first work block is January 4–24. Your time off is January 25–February 14. The next work block starts February 15.
If you import your work dates into a phone calendar, make a separate calendar for the rotation. That way you can replace the whole schedule if your start date changes.
Rotation Planner can generate a schedule from your own start date and export it to a calendar file or CSV. It costs $7 once and runs offline in a desktop browser.