Your promotion plan probably already lives in a spreadsheet. Now it can be the source of truth in twelfth too.
Under Settings → Calendars, share the sheet with the address shown and paste its link. twelfth picks the obvious tab, shows what each column reads as, and flags any row it can't read before anything is linked. Share it as Editor and twelfth can add a new, correctly laid-out tab for you in one click.
The sheet stays yours: twelfth reads it and never edits it after that first tab. Mention a calendar with @ in chat, or let twelfth consult it when timing matters. More in Calendars and on the calendars page.
