Before a monitor fires its first signal you have the option to backfill your signals. This means that you will include all previous instances that would have triggered your monitor before it was created.

You can do this by navigating to the monitor details page and click on the Backfill Signals button in the middle of the page.

If you do not see the button and see signals instead, you will not be able to backfill this monitor. If this happens but you would like to backfill, simply duplicate the existing monitor and backfill the new monitor before it fires. Make sure to delete the old one so you don't get extraneous alerts!