Diem adéu demà.

Catalan often uses the present tense for planned future events, just like English does in sentences such as We leave tomorrow.

So Diem adéu demà literally uses the present, but it naturally means We say goodbye tomorrow or We’re saying goodbye tomorrow.

Diem is the first-person plural present indicative form of dir, which means to say.

A few present-tense forms of dir are:

• jo dic = I say
• tu dius = you say
• ell/ella diu = he/she says
• nosaltres diem = we say

So diem tells you that the subject is we.

Catalan usually leaves out subject pronouns when the verb already makes the subject clear.

Because diem already means we say, you do not need nosaltres.

You can include nosaltres for emphasis or contrast:

• Nosaltres diem adéu demà, però ells marxen avui.
  = We say goodbye tomorrow, but they leave today.

Adéu means goodbye or farewell.

In the expression dir adéu, it works the same way goodbye does in English in to say goodbye.

So:

• dir adéu = to say goodbye
• diem adéu = we say goodbye

It is a very common and natural expression.

With dir adéu, Catalan normally uses adéu with no article. It is treated as a set expression.

So the natural form is:

• diem adéu

not usually:

• diem l’adéu
• diem un adéu

Those versions would sound unusual unless you meant a very specific or literary kind of farewell.

Because demà is an adverb meaning tomorrow.

Just like English says tomorrow without a preposition, Catalan does too:

• Arribo demà. = I arrive tomorrow.
• Diem adéu demà. = We say goodbye tomorrow.

So demà can directly express time on its own.

Yes. Catalan word order is fairly flexible, especially with time expressions.

These are all possible:

• Diem adéu demà.
• Demà diem adéu.

Both are natural. The second one puts a bit more focus on tomorrow.

A rough pronunciation guide is:

• diem ≈ DEE-em
• adéu ≈ ah-DEH-oo
• demà ≈ deh-MAH

A couple of useful notes:

• The accent in adéu shows the stressed syllable: déu
• The accent in demà shows that the stress is on the last syllable: mà

Exact pronunciation varies somewhat by dialect, but this will guide you well.

Yes. Ens acomiadem demà is also correct and means something very similar.

The difference is mainly in the verb:

• dir adéu = to say goodbye
• acomiadar-se = to say goodbye / to take leave

So:

• Diem adéu demà = straightforward, common
• Ens acomiadem demà = also correct, slightly different wording

For a learner, dir adéu is a very useful and simple pattern to remember.