Referring to the title, I have my first two campaigns that I'll be running very soon, and I've already drafted their respective final bosses. Is this a bad idea, considering the campaigns are in their very early stages at the moment? Again, I am very new to Dungeon Mastering, so I would appreciate any feedback you could give on the matter.

Nothing wrong with early prepping, you can always change them down the line.

Yes, in the sense that it is taking up time you could be spending building the first few level of your mega dungeon, when the party won’t encounter this boss for a long time given that they are all the way down on the bottom floor. 

I always encourage new DMs not to put the cart before the horse- that is, the BBEG before the dungeon.  Easy mistake to make when eager and excited and new to this sort of thing. 

Personally I think it's fine to have prepped the final bosses.  `You can always tweak them as you go - unless you've given the party loads of detail about whoever/whatever it is before hand - which is probably unlikely.  Even if the party has an idea of who the 'end game' is going to be, you can always make it surprising or more difficult or whatever by adding in additional surprises later on depending on how the campaign goes.

Also, having some idea of the final bosses means that you can start layering in clues or ideas about who or what that person or thing is. That can help build some anticipation, which isn't a bad thing I think.

Perhaps more important than who/what the final boss is though, is the motivation for getting them to that point - what is the driving force behind this encounter, why will the party be motivated to get there? That will be the thing that will keep the campaign going.

The thing to remember about planning ahead when running an RPG is that games can go in unexpected directions, and the further ahead you plan, the more likely that is that your plans turn into wasted work because they don't relate to the direction the game actually went. It's useful to have some idea of who the final boss is and what its evil plans are, but unless the PCs are complete failures, those plans are going to be mutating with time because the original plan ran into PC-shaped obstacles, and you may find yourself wanting to adjust the capabilities of the boss to make sure things come to a reasonable climax, instead of "guess the evil plan was foiled before we ever even met the boss because he didn't think of X".

Is it bad to prepare the final boss before the campaign has even started?

Not as long as you’re prepared for the facts that:

• You’ll likely have to change the fight when you finally get to it.
• It might never happen.

There is nothing wrong with preparing the BBEG at the beginning. Look at Curse of Strahd. It is right there in the title who the bad guy is. If you want, you use that adventure's strategy of presenting the bad guy near the beginning. Let the party meet him/her/it when they are massively underpowered and have no chance against them. Let the bad guy mock them and show them how powerless they are. That is actually a good thing as in serves as inspiration for the PCs to get more powerful, obtain the doohickey to defeat the bad guy, then hunt him down.

Of course you need to keep in mind that players will absolutely mess up your campaign. 

Quote from Wysperra >>

There is nothing wrong with preparing the BBEG at the beginning. Look at Curse of Strahd. It is right there in the title who the bad guy is.

Adventures for publication are very different from adventures for personal campaigns; yes, you can build your adventures like published adventures, but it's a ton of extra work that you will often not use and/or need to extensively modify in actual play.

I guess it depends by what you mean by "prepare"? If you mean just to pick out who they are, why they are doing whatever they are doing, where they are doing it, etc. I think that is a good excercise and should be done as part of your planning. That lets you have a clearer idea of how player actions would tie in, drop clues or hints more naturally, etc. If you are talking about planning the actual showdown, picking stats, building the dungeon (or wherever it is), populating minions etc., you are probably spending time that you would be better off spent building the stuff the players are going to encounter much sooner. Depending on how long of a campaign it is, you probably have a LONG time to iron out those details at more leisure, plus a LOT could change between now and then that would lead you to change and adjust stuff anyways.

For example in my parties current campaign the BBEG is Vecna, I know where he is, and what his ultimate plan is, but beyond that I haven't built anything about the showdown because the parties actions can (and have) lead to changes in things over all that would have just scrapped any final encounter I already had in mind, and at the rate these things take, and with my party lol, its probably at LEAST a year of semi weekly games before they even get in the general area of showdown with him, assuming they don't just go off in some wild direction and never wind up facing him at all.

i do this all the time. as long as you have good foreshadowing, it sould be fffffiiiiiiinnnnnneeeee

I assume you mean "prepare" as in developing the stats of the boss. If you mean narratively, it's of course good or borderline necessary to at least work out the general concept for a figure as important as the main villain relatively early on.

But as far as working out specific mechanics... eh. It's a statblock, and there's no harm in making a statblock. As the time to use it approaches, you may find that there are concerns like:

• Needing to change the challenge rating if the party is more or less powerful than you expected when you first designed the boss
• Needing to include counters for specific tactics the party uses a lot which could otherwise trivialize the fight
• Needing to balance the villain to fit other enemies that might need to appear in the fight

But, like, all of those are just things that might happen, and all of them are easier to deal with if you've already got something to start with.

And of course, getting through the next session is higher priority than long-term planning, but thinking ahead is useful too and certainly does no harm.