How long will homemade popcorn last?

It totally depends on how you top it and store it.

If you top with a butter or oil based topping, that will soak into the popcorn and make it soggy. The textural crunch is what tells you your popcorn is good and "fresh", so better to leave it untopped until "day of" if you are using a butter or oil type topping.

Caramel corn (plain, or further topped with chocolate and/or similar) is a different matter as the topping will harden too and help maintain the crunch. In this case, storage will be key. I keep caramel corn tight in a ziplock until wanted for use (which means using all at once, rather than unzipping and zipping again multiple times) up to two weeks (yep, two weeks - no preservatives). The key is to remove as much air as possible from the zip (use the straw trick, where you zip tight around a straw, inhale as much air as you can from inside the zip, and finish the zipping) and to store the popcorn ASAP once it has cooled (but let it cool).

I have not tried "storing" plain popcorn without toppings. I'm willing to bet you could make it a day or two before and store in zips (per above) and open/top/use on Saturday without anyone knowing the difference.

More popcorn second thoughts: Do you have a local health food store that sells things in bulk? Sometimes those places will have powdered cheese product (the type you'd find in a Kraft box dinner mac and cheese). And sell that in bulk bits. Which is great on popcorn.
You might also want to look into supermarkets that might have popcorn bags for individual servings--retro white/red stripes ones. Or simply have some small lunch bags available for easier serving. All depending on how many and situation of course.

Depending on where you are I have a flavoring recipe that's great!
It depends on a product called "true lime" which sold in little packets to flavor water etc and has a very pronounced lime flavor.

Sorry for the no measurements recipe here because it's rather to taste:
Kosher salt and few packets of the true lime ground up in a coffee grinder/spice grinder. Or just use as is.
To the lime salt mix add: garlic and onion powder; paprika; ginger powder; white pepper for heat (very much to taste).
Mix that with oiled or buttered pop corn by shaking in a large paper bag.

Here's a link to the crystalized lime powder website:
https://www.truelemon.com/true-lime
which has a store finder option.

The flavoring was inspired by the "Thai Kettle Chips" product which you might have seen or tasted.

It is best to keep it 2 days only, for the crunch&freshness of it. Definitely an airtight container, in the pantry- away from sunlight.
For the best results, i would definitely recommend making them in the morning of the bridal shower. Fresh is best :)

It really depends if you butter it or not . If u did then it will be no good for more than 2 days. If not i would say about 4 max