Insert custom fields based on criteria

Would be nice to insert custom fields by criteria (like season, arrival dates etc). 

Example: I want to include a paragraph in my response-to-inquiry email or my welcome email for winter travel, but only insert it if the booking falls in the winter time frame. 

I can create a custom field and insert it in correspondence, but it is inserted ALWAYS. It is not relevant to summer travel. 

Currently, I have a separate email for winter arrivals, but you know how renters are flooded with all kinds of emails....

I agree, it would be very nice to have custom fields attached to certain seasons.