If a National Forest campground allows reservations, in most cases, they also have a good number of first-come-first-served campsites. Reservable campsites may be reserved up to 6 months prior to your arrival date. Group campsites may be reserved up to 1 year in advance.
You may also reserve your campsite at least 4 days prior to your arrival. So, theoretically, you could reserve a campsite the week before Labor Day Weekend. However, the number of reservable campsites will be few and far between by that time.
If Labor Day is just around the corner, your best chance in securing a first-come-first-served campsite is to start your weekend a day or two early. To avoid last-minute disappointment, confirm with the Campground Manager that your site is a first-come-first-served site and cannot be reserved by someone else until you leave. Do not leave your campsite unattended for more than 24 hours.
Planning ahead: Labor Day for 2015 falls on September 8th; the prior Friday is the 5th So be ready to reserve a site in your favorite campground, on www.recreation.gov, on March 4th or 5th. You might want to start watching the availability a week or two prior; just to monitor your competition. Be ready to make a decision to change your arrival date in order to get a reservation.
Good Luck and Happy Camping!