You'll be downloading something called RunAsDate

This will not affect biome spread, and any pumpkins that grow during Halloween will (I think) be there if you go back to the normal date

Anything done to the bestiary will stay that way if you go back to the normal date

Worlds you make in that time will have that date even if you go the the normal time

If you wanna do research on RunAsDate you can, but I've used it and it hasn't ruined my life

RunAsDate lets you run any game at any time, you just need to download it.

Go on the website https://runasdate.en.softonic.com/ or search "runasdate download"
Once your here it'll look something like this (ads will pop up so I would suggest using an adblocker but it isn't needed)
Next you'll want to download it
Once you press Download, it'll take you here
Once you download it, it'll probably open this page, if the download didn't work you can relaunch the download but if it did, ignore this page

After you do the first part, you'll have it downloaded and have this pop up on your browser (If you closed it you can find it in your files)
Once clicking on it, it'll ask if you if you let this device make changes, click yes so it can install. (I couldn't get a screenshot)
You'll then see this as it downloads, once it finishes it'll download
DO NOT CLICK ANY OF THE OTHER STUFF THAT POPS UP WHEN INSTALLING, JUST WAIT FOR IT TO INSTALL, THE OTHER BUTTONS ARE ADS (Just click the X when it installs)
Once you have this in your files (This file shows books as I have winrar, winrar is not needed for this)






Open the folder and you'll find this (It looks a bit different as I have winrar)






Open the exe file and this'll pop up

Once you finish part one and 2, you should have this up



















Click on "Browse" and it'll show files, this is the path you'll want to take, I will show you how to get there.



Ignore me using Windows 11, if you follow the same steps you'll get there
Click on "This PC" and click on Windows (C:)






Click on "Program Files (x86)"































Find "Steam"































Find "steamapps"































Find "common"


















Find "Terraria"
































Once you click "Terraria" Double click the application named "Terraria" and you'll have it set to open Terraria

Once you finish the previous step, your RunAsDate should look like this




















Next you set you click the calendar and it'll open this, then you'll set your date to whichever holiday you wanna play in.




















Christmas is from December 15th - December 31st

Halloween is from October 10th - November 1st

Once you set your dates (I would go the 28th for both of them) You can set the time but it doesn't really matter

Next, press run, and it'll open at whatever holiday you set it to.




















And you're done, I hope this worked for you as it did for me, and just so you know, you'll have to open RunAsDate every time you wanna set your game to a holiday