Weird. That should have been fixed in the latest version 🤔
Did you check if the Portal Temple is still there? (The totem that let's you open the portal)
Try pressing F5 or Left Shift + F5.
Other solution might be to skip the days until the Xmas event appears again.
If anything works, you can try to get Nishy to LvL 20 and making a New Game +. You will start from 0 but keeping your Taming Progress and gaining a boost of EXP.
If anything works or need more help, you can join the Game's Discord Server through the main menu 👌