Στην Ελλάδα το Πάσχα το γιορτάζουν πολλές οικογένειες μαζί με τους συγγενείς τους.

Στην is the contracted form of σε την.

• σε = in / at / to
• την = the (feminine accusative singular)

So:

• σε την Ελλάδα → στην Ελλάδα

This contraction is very common in Greek. In the same way:

• σε το → στο

After the preposition σε (in / to / at), Greek normally uses the accusative case.

So Ελλάδα here is in the accusative:

• η Ελλάδα = Greece (nominative)
• την Ελλάδα = Greece (accusative)

That is why you get στην Ελλάδα = in Greece.

Also, Greek often uses the definite article with country names, unlike English.

In Greek, holiday names are very often used with the definite article.

So:

• το Πάσχα = Easter
• literally, something like the Easter

This is normal Greek usage, even though English usually says just Easter without the.

Also, Πάσχα is a neuter singular noun, so its article is το.

That το is not the article. It is the object pronoun it.

So:

• το Πάσχα = Easter
• το γιορτάζουν = they celebrate it

Greek very often repeats a direct object with a clitic pronoun, especially when the object comes before the verb. This is called clitic doubling.

So the structure is roughly:

• το Πάσχα = the thing being talked about
• το γιορτάζουν = they celebrate it

This is very natural in Greek.

Because the subject is πολλές οικογένειες = many families, which is plural.

• γιορτάζει = he/she/it celebrates
• γιορτάζουν = they celebrate

Even though πολλές οικογένειες comes later in the sentence, it is still the subject, so the verb agrees with it in number.

Greek word order is much more flexible than English word order.

In this sentence, the order is:

• Στην Ελλάδα = setting
• το Πάσχα = topic/object
• το γιορτάζουν = verb phrase
• πολλές οικογένειες = subject
• μαζί με τους συγγενείς τους = extra information

A more English-like order would be:

• Πολλές οικογένειες στην Ελλάδα γιορτάζουν το Πάσχα μαζί με τους συγγενείς τους.

But the original sentence is perfectly natural Greek. It gives a bit more emphasis to In Greece and Easter first.

Because οικογένεια (family) is a feminine noun.

So the adjective πολλός (many / much) has to match it:

• masculine plural: πολλοί
• feminine plural: πολλές
• neuter plural: πολλά

Since οικογένειες is feminine plural, the correct form is πολλές οικογένειες.

μαζί με means together with or along with.

• με by itself means with
• μαζί με adds the idea of togetherness

So:

• με τους συγγενείς τους = with their relatives
• μαζί με τους συγγενείς τους = together with their relatives

It sounds a little more explicit and warm/social.

Because the two τους do different jobs.

• the first τους is the definite article = the
• the second τους is the possessive = their

So:

• τους συγγενείς = the relatives
• τους συγγενείς τους = their relatives

This is completely normal Greek structure.

It is in the accusative plural, because it is the object of με in the expression μαζί με.

So:

• οι συγγενείς = the relatives (nominative plural)
• τους συγγενείς = the relatives (accusative plural)

In this sentence, μαζί με τους συγγενείς τους means together with their relatives.

γιορτάζω means celebrate, so it can be used for many kinds of events, not just holidays.

For example:

• γιορτάζω το Πάσχα = celebrate Easter
• γιορτάζω τα Χριστούγεννα = celebrate Christmas
• γιορτάζω τα γενέθλιά μου = celebrate my birthday

So in this sentence, it is simply the normal verb for celebrate.