BRTD, I also wouldnt necessarily conflate people's dress for a first Holy Communion to their normal church attire. Most people on special events step up their dress as a sign of respect and as a symbol that it is indeed a special occasion. This is really the norm across most cultures and religions.
But when I got baptized and we also did our infant dedications, the only people dressed up were olds who obviously weren't normal attendees and were mostly family of the parents of the babies being dedicated. The parents and pastors (and of course me) were still all as casual as normal.
This is also why most people don't dress like they just rolled out of bed when they go to church.
I'd dispute the "most" in the modern day. It's not 50 years ago.
Your own life experience is not the baseline for all of humanity.Just because you and your peers like hardcore shows or go to church in a T shirt does not mean that 1. it matters at all or says anything meaningful about said concert goer or T shirt wearer 2. that anyone else should care 3. This and the several previous threads or discussions on the matter betray a level of superficiality and seems a conceit. The, I couldn't bother to get dressed up, is, to me at least, tiresome and immature. It is all a pose. Ive seen so many "scene" people who likely spent hours on their messy hair.
Further, By dwelling on this so much it is all quite obvious that you intensely care on what you and others are wearing. Hence, my sarcastic suggestion that you get your own fashion sub board. The fact that you are concentrating so much on what others around you are wearing rather than the message you are receiving makes you no better than the blue haired old ladies who snicker at another woman's scandalous top or skirt.
Lastly man, you are an old! I'd hope that obsessing over a little clique and again its conceits (your own uniform and soundtrack) is something most people leave behind in high school or latest college. Wear and listen to what you like but don't think that it says something really transcendental about you as a person. I'm glad you have found a group of people you like to worship with, however, the costume you do it in is really insignificant.
So to answer your OP, nothing.