Until then vs. by then

[olegv]

Please explain to me the difference between until then and by then in the following sentences.

I can't leave on Tuesday. I won't be ready until then/by then. Thank you.

 

[5jj]

Please explain to me the difference between until then and by then in the following sentences.

I can't leave on Tuesday. I won't be ready until then/by then. Thank you.

With 'until then', you will be ready on Tuesday. With 'by then', you will not.