That’s understood but there’s no Date Trigger in Home Assistant. What kind of trigger are you planning to use?
For example, we can have a Time Trigger trigger at a specified time and then use a Template Condition to check if the current date is 21 days before the sensor’s date. Or we can use a Template Trigger to trigger at a specific time and 21 days before the sensor’s date. In both cases, the triggers require a time.
The sensor just have a date no time. So then easy is to check every day and condition must be 21 days before the date of sensor. Then it will fire the action.
The following Template Trigger evaluates the template every minute. When the current date is equal to the sensor’s date less 21 days, it will report true and trigger. This will occur at 00:00 because that’s the starting time of each new day.
If you don’t want it to trigger at 00:00 then you will need to tell me at what time you want it to occur (and then the Template Trigger will be replaced by a Time Trigger and Template Condition).