Instead of starting a campaign using a scheduled date and time, you can use a trigger to send relevant content to individual contacts, at the right time for them.
In the case of the "Tag Changed" trigger, you can send content based on a tag being Added, or Removed from a contact's profile. *

* Tags may be added or removed manually; when synced with categories from your integrated CRM; or automatically based on intent. "Tag Changed" is not available to Office Wide users.

Once the campaign is activated, if the chosen tag is added or removed (as set) for any contact, the contact will receive the campaign email. *

If you set the trigger to tag Added, you also have the option to send to contacts who currently have the specified tag (1), not just those that have the tag added after activation. When the campaign is activated, contacts that already have the tag receive the first email straight away. *

* As long as they match any other conditions that may have been added before "Send Email" .

Only one tag can be specified here. If you have several tags with similar names, e.g.: "Buyer" and "Buyers", you may want to merge them, so that you target all relevant contacts.

When using a trigger to start a campaign like this, Recipients should be set to "Send to All", as the Tag Changed trigger also acts as a filter. You're including all your contacts, but only sending to them if and when the content becomes relevant to them.

* Learn more about How to use Tags in a campaign