Postmorgaŭ, la tria de marto, ni renkontiĝos en la kafejo post la laboro.

Postmorgaŭ means the day after tomorrow.

It is built from:

• post = after
• morgaŭ = tomorrow

Esperanto often combines elements like this into a single word when the meaning is clear and common. So postmorgaŭ is a normal standalone adverb of time.

Esperanto commonly expresses dates with:

• la
  • ordinal number + de
    • month

So:

• la tria de marto = the third of March

You could also write the number as:

• la 3-a de marto

Both are normal.

In Esperanto, ordinal dates normally use la:

• la unua de majo = the first of May
• la deka de junio = the tenth of June

So la tria de marto is the standard pattern.

In dates, Esperanto uses de between the day number and the month:

• la tria de marto
• la dek-kvina de aprilo

This works like the third of March in English.

You use en marto when you mean in March in a general sense, not a full date:

• Mi veturos en marto. = I will travel in March.

Because la tria de marto is extra information that explains postmorgaŭ.

The structure is basically:

• Postmorgaŭ — that is, the third of March — we will meet...

So the date is in apposition: it restates the same time in a more precise way.

The commas show that it is an inserted clarification.

This is a very common learner question.

• renkonti = to meet someone / encounter someone
• renkontiĝi = to meet each other / to get together / to come together

So:

• Ni renkontiĝos = We will meet
• Ni renkontos... usually sounds like We will meet/encounter someone or something, so it often expects an object

For two or more people arranging to meet one another, renkontiĝi is the natural verb.

It can be broken down like this:

• renkont- = meet
• -iĝ- = become / get into a state / become intransitive
• -os = future tense

So renkontiĝos means something like will come together / will meet.

The most important thing for a learner is simply that renkontiĝi is the standard verb for to meet each other.

Because en shows location, while al shows movement toward something.

• en la kafejo = in the café / at the café
• al la kafejo = to the café

In this sentence, the meeting happens there, so en is the right choice.

Compare:

• Ni iros al la kafejo. = We will go to the café.
• Ni renkontiĝos en la kafejo. = We will meet in the café.

Esperanto often uses the article where English does not.

Here la laboro means something like:

• the work
• the workday
• the job/work we both know about

So post la laboro is a natural way to say after work.

English often drops the article in expressions like after work, but Esperanto does not always do the same thing.

A date can sometimes take the accusative -n when it functions directly as a time expression:

• Ni renkontiĝos la trian de marto. = We will meet on the third of March.

But in your sentence, la tria de marto is not the main time expression by itself. It is explaining postmorgaŭ.

So the structure is:

• Postmorgaŭ, la tria de marto, ...

Because it is an explanatory apposition, the nominative la tria is perfectly natural here.

The word order is fairly flexible.

You could also say:

• Ni renkontiĝos postmorgaŭ, la tria de marto, en la kafejo post la laboro.

Putting Postmorgaŭ first gives the time immediately and sounds very natural. Esperanto often moves elements around for emphasis or style, as long as the grammar remains clear.

They do say the same thing, but that is not strange.

Using both is helpful when you want to:

• make the timing extra clear
• confirm the calendar date
• avoid misunderstanding

It is similar to English:

• The day after tomorrow, March third, we’ll meet...

So it is redundant in a useful, clarifying way, not in a bad way.