Η προθεσμία ολοκλήρωσης είναι αύριο.

Η is the feminine singular definite article, so it means the.

It matches προθεσμία, which is a feminine singular noun. So:

• Η προθεσμία = the deadline

Greek articles have to agree with the noun in gender, number, and case.

προθεσμία is the subject of the sentence, and it is in the nominative singular.

So the basic structure is:

• Η προθεσμία ολοκλήρωσης = the completion deadline
• είναι = is
• αύριο = tomorrow

Because προθεσμία is the thing being described, it appears in the nominative.

Because it is in the genitive singular.

The dictionary form is:

• η ολοκλήρωση = completion

But after another noun, Greek often uses the genitive to mean of:

• ολοκλήρωσης = of completion

So:

• προθεσμία ολοκλήρωσης literally means deadline of completion

In natural English, that is usually completion deadline or deadline for completion.

Greek often leaves out the article in compact noun phrases like this, where the second noun describes the type or purpose of the first noun.

So:

• προθεσμία ολοκλήρωσης = completion deadline / deadline for completion

If you say της ολοκλήρωσης, it sounds more specific, as if you mean a particular completion already known from context:

• η προθεσμία της ολοκλήρωσης

That is possible in some situations, but it is less neutral here. The version without the article is the more natural general phrasing.

είναι means is.

It is the 3rd person singular form of είμαι = to be.

So the sentence is built just like English:

• Η προθεσμία ολοκλήρωσης = the completion deadline
• είναι = is
• αύριο = tomorrow

Because Greek can use time words like αύριο directly after είναι, just as English can:

• The deadline is tomorrow

No preposition is needed.

A slightly different version is:

• Η προθεσμία ολοκλήρωσης είναι για αύριο

That means something like The completion deadline is for tomorrow or is due tomorrow. It is also natural, but your original sentence is completely correct.

In this sentence, αύριο functions as a time expression meaning tomorrow.

For a learner, the most important thing is that Greek uses it very naturally in sentences like:

• Η συνάντηση είναι αύριο. = The meeting is tomorrow.
• Το μάθημα είναι αύριο. = The class is tomorrow.

So even if English learners expect a preposition, Greek does not need one here.

Yes. Greek word order is more flexible than English.

The version you have is the most neutral:

• Η προθεσμία ολοκλήρωσης είναι αύριο.

But you could also say:

• Αύριο είναι η προθεσμία ολοκλήρωσης.

This puts more emphasis on tomorrow.

The meaning stays basically the same, but the focus changes depending on what the speaker wants to highlight.

A rough pronunciation guide is:

• ee pro-thes-MEE-a o-lo-KLEE-ro-sis EE-ne AV-rio

A few useful notes:

• Η sounds like ee
• είναι sounds like EE-ne
• αύριο sounds like AV-rio
• the stress falls on -μία, -κλή-, εί-, and αύ-

This is only an approximation, but it is a good starting point.

Yes. It is natural, correct, and neutral.

It works especially well in:

• work contexts
• school or university contexts
• emails
• formal or semi-formal speech

In everyday conversation, people might sometimes shorten it if the context is already clear:

• Η προθεσμία είναι αύριο. = The deadline is tomorrow.

Or they might express the same idea differently:

• Πρέπει να το ολοκληρώσουμε μέχρι αύριο. = We have to complete it by tomorrow.

But the original sentence sounds perfectly normal.