Δεν ξέρω αν είναι να έρθει σήμερα ή αύριο, γιατί τυχαίνει να έχει πολλές συναντήσεις αυτή την εβδομάδα.

Not really, at least not primarily. The sentence is best understood as:

Δεν ξέρω αν... = I don’t know whether...

and then inside that clause:

είναι να έρθει = is supposed / expected / due to come

So αν belongs with δεν ξέρω, while είναι να έρθει is its own structure.

This is an idiomatic pattern: είμαι να + verb.

It often means:

• to be supposed to
• to be expected to
• to be scheduled to
• to be due to

So είναι να έρθει is not just a plain future like will come. It suggests some plan, arrangement, expectation, or scheduled idea.

Because after να, Greek normally uses the subjunctive.

So:

• να έρθει = subjunctive
• έρχεται = indicative present

Also, έρθει is the aorist subjunctive form of έρχομαι. Here the aorist does not mean past time. It shows perfective aspect, meaning the coming is viewed as a single whole event.

So να έρθει fits the idea of to come on one occasion, not an ongoing or repeated action.

Because θα έρθει and είναι να έρθει are not exactly the same.

• θα έρθει = he/she will come
  This is a straightforward future statement or prediction.

• είναι να έρθει = he/she is supposed / expected / scheduled to come
  This adds the idea of arrangement, expectation, or plan.

So the sentence is not just talking about future time; it is talking about when the person is supposed to come.

Τυχαίνει να + verb is a common expression meaning:

• to happen to
• it so happens that

So τυχαίνει να έχει πολλές συναντήσεις means something like he/she happens to have many meetings.

The verb τυχαίνει comes from τυχαίνω, which is related to things happening by chance or coinciding.

Because τυχαίνει is commonly followed by να + subjunctive.

So the pattern is:

τυχαίνει να κάνει κάτι = to happen to do something

With έχω, the 3rd-person singular present subjunctive looks the same as the present indicative:

• indicative: έχει
• subjunctive: να έχει

So the important marker here is να.

Greek very often leaves subject pronouns out when they are understood from context. This is normal and very common.

So instead of saying: Δεν ξέρω αν αυτός / αυτή είναι να έρθει...

Greek simply says: Δεν ξέρω αν είναι να έρθει...

The verb form tells you it is 3rd person singular, and the context tells you who that person is.

πολλές συναντήσεις means many meetings.

Grammatically:

• συναντήσεις is the accusative plural of συνάντηση
• πολλές agrees with it in gender, number, and case

This is the direct object of έχει, because the person has many meetings.

Because this is a time expression, and Greek often uses the accusative for expressions like this week, this month, all day, and so on.

So:

• αυτή η εβδομάδα = this week as a subject
• αυτή την εβδομάδα = this week as a time expression, meaning during this week

That is why both αυτή and την appear in the accusative feminine singular form.

Here γιατί means because and introduces the reason clause.

So it explains the reason for the uncertainty.

Be careful: γιατί can also mean why in questions. Greek uses the same word for both:

• Γιατί έφυγες; = Why did you leave?
• Έφυγα γιατί κουράστηκα. = I left because I got tired.

In this sentence, it clearly means because.

Because σήμερα and αύριο are adverbs of time, like today and tomorrow in English.

So Greek simply says:

• σήμερα = today
• αύριο = tomorrow

No article is needed.

The clause γιατί τυχαίνει να έχει πολλές συναντήσεις αυτή την εβδομάδα explains why the speaker doesn’t know whether it is today or tomorrow.

In other words, the many meetings make the schedule uncertain. So the reason clause is connected to Δεν ξέρω more than to the act of coming itself.