Hey there!
So, if you get it from the thread title, there's a Halloween event every year, or the least it's supposed to be there.Well, this is just a rough idea about it, and it could be helpful, not sure of that.
Basically, the halloween event is nothing but a minigame, where you're supposed to get candies via trick/treating and then spend them on getting coloured pokemons.

To begin with,you're gonna need a couple items and pokemons, I've added them in the spoilers below:

I got all the item and pokemon, now what?
Nice, now to actually have access to the old house in your city, you need to get a Dream Eater trainer style which can be edited from your account preferences.
Once you have the style, the old house will have a password which is pretty easy to guess, in case you can't you can simply ask people in the chatroom.
Now that you can visit the old house, you need to get a few costumes for trick/treating first, right?
This old house actually belongs to a witch who'll give the costumes you need.

However, the witch doesn't simply give you the costumes, she wants certain items and pokemons, which I mentioned earlier. She doesn't take away the items or the pokemons, you can loan them from someone and return them in case you don't want to pay the hefty price for the TM, or in case you don't have the pokemons.

Great, now I have all the costumes! What do I do next?
Once you've collected all the costumes, you have to visit each and every city by moving through the SS Anne and Airport to travel to different regions and cities.
In these cities, you have to trick/treat the people of the city, basically to get the candies.
To trick/treat the city, there's a link to the bottom of the old house link you clicked earlier.
You can only trick/treat the city once, but everytime you trick/treat, you'll get 1-5 candies.

Well, err, why is there a trick/treat option on every user's profile? Has it got anything to do with the event at all?
Trick-or-treating is also a minigame that you can see on every user's profile page.You can either "trick" or "treat" each member, which will either cause you to gain/lose in-game money, or gain/lose candies.
"Tricking" and "treating" lead to exactly the same result, so it doesn't really matter which one you choose. This can only be done on each member once per day, and is also a good way of collecting candies.

Now all set, what do I do with these candies?
That would be a really stupid question to ask, but stupid is life :)
So you can basically buy prize pokemons for the candies. There are 4 visible prizes, basically coloured pokemons, and they're supposed to be halloween themed, but they are somewhat decent podgeys.

Besides these 4 podgeys, there's a 315 candy mysterious prize, and this prize is a coloured legendary pokemon that hasn't yet been added to the game.
Once you're done with trick/treating all the cities, most of the people do have enough for the 315 candy prize, but in case you don't, you can just go for the 50 candy prizes or trick treat a couple players to get to 315 candies.

Well, I'm pretty sure it takes a lot of efforts to actually get ot 315 candies, but I'd say that the pokemon's worth it.
Hope this helped you with the event.

Happy Halloween!