Termenul de restituire este vineri.

Because Romanian puts the definite article at the end of the noun.

• termen = a deadline / a term
• termenul = the deadline / the due date

So termenul de restituire means the return deadline or the due date for returning.

In this sentence, termen means something like deadline, due date, or time limit.

The exact English choice depends on context:

• library book: due date
• official document or formal context: deadline or return deadline

So Romanian termen is a flexible word and often sounds a bit more formal than everyday English due date.

De restituire literally means of return or for returning.

Here:

• restituire = returning / restitution / handing back
• de links it to termenul

Romanian often uses this kind of structure where English might use:

• a noun + noun combination, or
• an adjective-like phrase

So:

• termenul de restituire = the return date
• literally: the deadline of returning

Because in Romanian, after de in this kind of fixed noun phrase, the second noun is usually left indefinite.

So:

• termen de restituire = return deadline
• termen de plată = payment deadline
• sală de așteptare = waiting room

If you said restituirea, that would sound more like a specific, already identified act of returning, not the general idea of return as a category.

So de restituire is the normal form here.

Because Romanian often uses the weekday name by itself.

• Este vineri. = It is Friday.
• Termenul de restituire este vineri. = The due date is Friday.

Romanian also often uses the bare day name for on Friday:

• Vin vineri. = I’m coming on Friday.

So unlike English, Romanian does not always need a word like on before a day of the week.

It is the name of the day, Friday, but in Romanian weekday names often behave a bit like time expressions too.

In this sentence, vineri functions as the complement after este, telling you what day the deadline falls on.

So the simplest way to think of it is: it is the weekday name Friday, used in a very natural time expression.

This is an important difference:

• vineri = Friday / on Friday
  usually one specific Friday, depending on context

• vinerea = on Fridays / every Friday
  often habitual or repeated

Examples:

• Termenul este vineri. = The deadline is Friday.
• Biblioteca se închide vinerea. = The library closes on Fridays.

So in your sentence, vineri is correct because it refers to a specific day.

Yes. Very often.

• este = full form, a bit more careful or formal
• e = shorter, very common in speech and informal writing

So both are correct:

• Termenul de restituire este vineri.
• Termenul de restituire e vineri.

The meaning is the same.

Yes, Romanian word order is fairly flexible.

The neutral version is:

• Termenul de restituire este vineri.

But you may also hear:

• Vineri este termenul de restituire.

That version puts more emphasis on vineri.

So the original sentence is the most straightforward, but other orders are possible depending on emphasis.

Yes, it sounds somewhat formal or administrative.

You may see it in contexts like:

• libraries
• contracts
• official notices
• legal language

In more everyday Romanian, people might sometimes prefer returnare in some contexts. But termenul de restituire is completely correct and very natural in formal usage, especially for borrowed items, documents, or official returns.