Hodiaŭ estas la unua de marto, sed antaŭhieraŭ estis ankoraŭ februaro.

Esperanto often does not use a dummy subject like English it in expressions about time, dates, weather, and similar things.

So:

Hodiaŭ estas la unua de marto.
literally works without any separate subject.

Here, hodiaŭ means today, but it is an adverb, not a grammatical subject. The same idea applies in:

Antaŭhieraŭ estis ankoraŭ februaro.

Using ĝi here would usually sound unnatural.

Because dates use an ordinal number, not a cardinal number.

• unu = one
• unua = first

So la unua de marto means the first of March.

If you said unu de marto, that would not be the normal way to give a date.

The ending -a marks an adjective in Esperanto. Ordinal numbers are formed as adjectives:

• unua = first
• dua = second
• tria = third

In a date, la unua is really short for something like la unua tago (the first day). The noun tago is just left out because it is understood.

In Esperanto, dates are normally expressed with the ordinal:

• la unua de marto
• la dua de aprilo
• la dek-kvina de junio

So la is part of the normal date formula. It is attached to unua, not to marto.

This is the standard Esperanto way to connect the day and the month:

la unua de marto = the first of March

So the pattern is:

la + ordinal + de + month

Examples:

• la tria de majo
• la dudeka de novembro

It is very similar to one common English pattern: the first of March.

Yes. That is completely normal.

You can write:

• la unua de marto
• la 1-a de marto

Both mean the same thing. The -a is important because it shows the number is ordinal (first) rather than cardinal (one).

Because here februaro is being used as a predicate noun after esti:

estis ankoraŭ februaro = it was still February

That identifies what month it was.

If you said en februaro, that would mean in February, which usually places some event within that month:

• Mi naskiĝis en februaro. = I was born in February.

So:

• estis februaro = it was February
• estis en februaro = it was in February

Those are different ideas.

Ankoraŭ means still here.

So:

estis ankoraŭ februaro
means that at that earlier time, March had not arrived yet.

Depending on context, ankoraŭ can also mean yet, but in this sentence still is the natural sense.

In Esperanto, word order is fairly flexible, and ankoraŭ is often placed near the part it modifies.

Here:

estis ankoraŭ februaro

puts the focus naturally on the idea that it was still February.

You may also see:

ankoraŭ estis februaro

That is also understandable, but the original version is very natural.

Yes. Antaŭhieraŭ is a single Esperanto word meaning the day before yesterday.

It is built from:

• antaŭ = before
• hieraŭ = yesterday

Esperanto has several handy time words like this:

• hieraŭ = yesterday
• hodiaŭ = today
• morgaŭ = tomorrow
• antaŭhieraŭ = the day before yesterday
• postmorgaŭ = the day after tomorrow

Because in Esperanto, names of months are normally written with lowercase letters:

• januaro
• februaro
• marto

This is different from English, where month names are capitalized. The same lowercase rule usually applies to days of the week and language names as well.

The letter ŭ is a short glide, not a full vowel by itself. In aŭ, it sounds roughly like the ending of English now or cow.

So:

• hodiaŭ ends roughly like ow
• antaŭ also has that ow sound
• hieraŭ ends the same way

It is best to think of aŭ as one smooth diphthong rather than two fully separate sounds.