The date range you choose must be the exact checkin and checkout dates of your desired trip.
When checking the availability for your alert it will not match and alert you unless ALL of the dates (except checkout, of course) in your date range are available.
It is not currently possible to enter a large date range and get an alert if some dates in that range become available.
For example, if you want to camp for the 4th of July holiday here are some valid and invalid examples of date ranges for alerts:
Valid
Alert Dates Selected: July 2nd-July 6th
Campsites Available: July 3rd, July 4th, July 5th - No Match, No Alert
Campsites Available: July 2nd, July 3rd, July 4th, July 5th - Match, Alert Sent
Invalid
Alert Dates Selected: July 1st-July 31st
Campsites Available: July 2nd, July 3rd, July 4th, July 5th - No Match, No Alert
Campsites Available: July 10th, July 11th - No Match, No Alert