There's Halloween furniture, thanksgiving furniture and Christmas furniture!!
Halloween is the hardest of all to get - and it's a real pain. You need to start at the beginning of October and go to Tom Nook's store each day and buy a piece of candy. Store up as much candy as possible. At 6pm on 10/31, all the towns residents come out to go trick or treating. All of the towns residents are dressed the same (with pumpkin heads) so you can't tell them apart. They all run around and wobble their heads - but there is one 'resident' that doesn't - he stays very still and just hangs out in weird places. That one is Jack and that's the one you need to find. Run away from all the others, because they will stop you and demand candy. If you refuse, they will turn an item in your 'pockets' into a jack in the box (which is okay if you have items you want to sell or get rid of, because a jack in the box sells for about 1,000 bells).
You need to make sure your pockets are completely empty first, except for one piece of candy. To save time, I 'hide' the rest of my candy in letters as presents so I'm not running back and forth to my house to get another piece each time I find jack. If you find Jack and give him a piece of candy, he'll reward you with Halloween furniture. There are several pieces so you have to do this over and over again (all the while trying to avoid the other residents) in order to get the entire set (and there is A LOT of furniture). The furniture sells for big bells at Tom Nooks, and you could easily earn enough for a house upgrade or two depending on how lucky you are. Remember - Jack is always VERY still and once you find him, it's usually at least 10 or 15 minutes before he'll reappear.
For the thanksgiving furniture, you need to time travel to Thanksgiving day then go to the town square and talk to the Mayor. He'll give you a knife and fork - then run around the town and look for the turkey that's usually hiding behind buildings, afraid of being eaten. Each time you give him the knife and fork, he'll give you a piece of harvest furniture (also worth big bells).
I forgot how the Christmas one goes, but I'll post the tips for that when I figure it out again
If I remember correctly, it has to do with constantly changing outfits then finding a reindeer or something like that
I really can't believe I'm typing this in public - I'm never going to live it down.
Pete