Is "by" needed when I use "until"?

I wonder what is the correct sentence and I hope you can help me:

1. This will be done until the end of this year.
2. This will be done until by the end of this year.

Is sentence 1 or 2 right? Or are both possible?

Done is an individual action or cessation of action, and until implies a period of time. So your first example would only be grammatically correct in the context of "My set of commemorative coins will be complete until the mint releases a new one next year." I cannot find a grammatically-correct use case for the second example.

Now, if you said, "This will be done by the end of this year.", you'd be giving yourself the period between now and the end of the year with which to complete a task. If you said, "This will be done at the end of this year.", you'd be implying that for whatever reason, you're waiting to do this task until around year-end. Likewise, if you said, "This will be worked on until the end of this year.", then you'd be implying that this task will take months of constant work to complete.