So. Opet.
This would be a New Kingdom festival. The Medinet Habu calendar has it beginning on 2 Akhet 19; it lasted between 11 and 27 days, apparently, possibly running longer in some times due to the desire of post-Amarna NK kings to reaffirm their connections with Amun, or due to the complex shifts of power and control between the kingship and His priesthood.
In the festival, the icons of the gods known as the Theban Triad (Amun, Mut, and Khonsu) would travel from their residence at Karnak to the capital at Thebes. There, they would be greeted by the king, and coronation rituals would be reaffirmed. This official visit from the divine dignitaries served to reaffirm the connection between Amun as the state deity and father of the king and the mortal world, thereby making sure the cosmic order and the temporal order were properly synchronised. At the same time, the population would have time for feasting and celebration.
This is a fairly straightforward festival when it has the context of an established state in which it can take place, where the responsibility for the connections between the cosmic and temporal orders devolves upon the specific figure of the king, whose orders and actions then propagate that connection out into the world as a whole. The iconic symbolism all works quite tidily there, back where it started, with all the loose ends tied up neatly.
With the dissolution of the polytheistic Egyptian state as a religious entity, the whole thing gets a whole lot fiddlier, of course; with the followers of the netjeru scattered across continents, speaking many languages, rarely able to gather together in any numbers, the whole thing gets quite messy. There is no single linchpin from which all things follow anymore; even if one has chosen to join a temple that has a king, there are still other temples with other kings, and people who have none.
But. Opet.
First of all: Opet is a restoration of the connection between the cosmic order and the temporal one. This was achieved, first, by the state triad paying an official state visit to their mortal offspring, the king, and secondly by their witness and participation in the reaffirmation of the king's position as head of state and incarnation of the cosmic linchpin. Thus, people marking Opet need to reaffirm their position in this whole cosmic-temporal order. This is a good time to make and reaffirm spiritual vows, to return to commitments that have fallen neglected over time, to restore to order what has lapsed. Amun must also be welcomed, the connection between the cosmic and the temporal reaffirmed with ritual, and the relationship between the human and the divine maintained.
Secondly: The core mechanism of this affirmation is a relationship, the parental image of Amun visiting His son. Relationships are the means of propagating ma'at. Not only would it be appropriate to affirm connections with Amun, but with other patron or parental deities, and further, with other human beings. We are part of communities. Participating fully in those communities strengthens both us as individuals and the communities we are a part of. Since Kemetic religion is now a diaspora religion, it is unlikely that those communities will be of co-religionists, but that does not matter. These are the communities, the families, the forms that give us all support, structure, affirmation, and reinforcement, and it is time to support and affirm those places as well.
Thirdly: In ancient times, the state sponsored a feast accessible to all attendees. Obviously, very few of us are going to have our local governments shell out for a party; however, we can affirm that spirit of support and sustenance beyond our ordinary communities as well. I would put in a word for volunteering, charitable donations, and working in support of the availability of such resources, especially as texts from the First Intermediate Period on indicate that officials considered their provision of basic necessities to their populace to be proof of their status in the cosmic order as proper delegates of the royal ka. September is early for the standard United States charitable gifting season (of November and December), but such organisations require help year round, and I think it's worth it to note that we, too, have religious holidays that can encourage charitable work.
(I have thoughts about the structure of the Theban Triad and possible symbolic extrapolations regarding it, but I think those can go in a separate discussion.)