I'm sorry to hear this. It sounds very hard.
As another poster asked- are you actively looking for another home? That is the number one priority - no matter what some people are saying here, it might be in 2 months, it might be in 6 months, but you are going to have to leave at some point, so it's obviously much better for your mental health if you have something sorted.
Again, another posted asked if your parents could take in your pets temporarily - great idea.
Right, the legalities. The section 21 expires in December, so that's the point they are asking you to leave. They can't make you leave at that point. There won't be bailiffs at that point. That is the point that they can apply to the court to get you evicted. That will take months to do. It's an expensive process for the landlord, it's an uncomfortable process for you, as you have no idea what's happening and no control.
What I would do:
Talk to the council, depending on where you live the situation isn't always awful- there may be properties available. Look for private properties asap, assess your options. Is the local market still in your budget? Do you work? Are you likely to qualify for a rental? Ask on Facebook groups, ask friends, you never know where you're going to find something.
When you know what your situation is, talk to your landlord. Assuming that they are a decent enough person of course, given you've been there a long time. Let them know that you know when your section 21 runs out, but that you might not be able to meet the deadline, but you are planning to keep paying rent. Ask them how flexible they can be for things like notice- if you found a property next week, could you take it and stop paying rent? Can they refund your deposit early if it helps you financially to secure somewhere? Do they know of any other properties that might be available? What are the plans for the property- if they're selling, are you in a position to buy, would they offer you a discount?
Once you know these things, you might be in a little bit of a stronger position, or at least less worried.
Finally, DO NOT just move out at Christmas if you don't have somewhere. You'll put yourself in the worst possible position.
I really am sorry you're going through this. I'm going to presume it's another unintended consequence of the renters reform, but you are the person suffering. I hope there are properties nearby for you.