Is there any special etiquette or "good things to know" for hiking a section of the Appalachian Trail (AT) during AT, or peak, season (for that section)?
I think what you are trying to get at is the proper etiquette for hiking on the AT during peak season, when the most hikers and backpackers are on the trail. Exactly when peak season is, and how many people you will see at peak season really depends on where you are on the trail. Various places will have different number of Thru-hikers, section-hikers, weekend backpackers, and day hikers.
My experience is mostly hiking the parts of the AT in Virginia and North Carolina. We see Northbounders and Southbounders cross as well as lots of day and weekend activity.
Etiquette I would break into things you really need to be aware of and just nice things to do. As a day hiker I would say to be very aware of the following:
As for things I find that are good to do, and I would consider good etiquette are:
Also, you mention hiking, but there is one important etiquette item I would want to add for backpackers during peak season for thru-hikers. Bring your own tent/tarp, even if you expect to stay near a shelter. If more hikers end at a shelter than the shelter can hold I always stay in my own tent to make sure thru-hikers get the shelter.
In peak season (late spring/summer) most trail etiquette on the AT relates to thru-hikers, but not all of it, and generally is about the same on the whole trail. Thru-hikers are of course those who are continuously hiking the entirety of the Appalachian Trail, north or southbound. This information is essentially entirely based on my experience on the AT and hiking in general for many years.
In short, simply be polite and respect others and the wilderness in which you find yourself. Leave things better than you find them.
In more detail, general etiquette is the following:
For information on specific sections I recommend a guidebook for the particular section being hiked. There are several available. The Appalachian Trail Conservancy offers these guidebooks which are quite detailed, and may be found in many outdoors stores near the AT as well as online. Another kind with a digital version is offered here from the ALDHA. Downloads are free for ALDHA members only. It seems you'll have to pay for pretty much any type of guide, but it's worth it usually. While the primary purpose of these guidebooks is mostly to educate about the trail and the area, you could likely infer specific etiquette from that information if needed.
And finally, go to a local (not a chain) outfitter/outdoors store in the area you're visiting. They will likely have good information and be up-to-date on the local trails. Also many regions have specific hiking clubs and associations that maintain websites with a good amount of information and people to contact.