If I’m understanding you correctly, then the “Publish Date” and “Unpublish Date” settings are exactly what you need. You can set a page to publish at any future date and time and to unpublish at a later date and time, MODX will automatically set the page’s “published” setting accordingly when that date and time arrives. So, for instance, if you have an event running from June 7 – June 9, you can set the Publish Date to June 7 at any time you want the page to become available and set the Unpublish Date to June 9 at any time you want the page to cease being available.
Just be sure you check your server’s timezone setting to ensure the times you’re using match the times the server will be using.