Which is correct here: "*sometime* next month" vs. "*sometimes* next month"?

Are those expressions equivalent, or which one should be preferred?

For instance:

I should finish this work sometimes next week

versus

I should finish this work sometime next week

Although they look similar and are both adverbs, they are not equivalent.

• Sometime means "someday, at some time, at some point; during, in the course of.", and it's the word you want to use in this case.

I should finish this work sometime/at some time/during next week.

As you can see, replacing it, it still makes sense.

• Sometimes, instead, means "occasionally, from time to time, now and then, etc."

If you try to replace it now, it won't make sense:

I should finish this work sometimes/from time to time next week.

This last one should be used for example to say:

Sometimes I like taking a walk. (which becomes) From time to time, I like taking a walk.

sometime means "at a single unspecified point in time."

sometimes means "at various unspecified points in time."

When you say "I should finish this work..." you're talking about specific work, and it's only logical that said work can only become finished at one point in time, so only "sometime" makes sense.