Learn Hardcore Portuguese (Brazil): É importante que o relatório seja feito antes da reunião. - It is important that the report be done before the meeting.

Because é importante que... is an expression of necessity/importance and it triggers the present subjunctive in the following clause. So you get que o relatório seja feito (subjunctive) rather than que o relatório é feito (indicative). In English you often use should or a “mandative” structure: It’s important that the report (should) be done....

What exactly is seja feito grammatically?

Seja feito is the present subjunctive of ser (seja) + past participle feito (from fazer). Together they form a passive voice: to be done / to be made.
Active equivalent: É importante que (alguém) faça o relatório... (It’s important that someone does/writes the report...).

Why use passive voice here instead of saying who does it?

Portuguese often uses the passive when the doer is unknown, irrelevant, or intentionally omitted. Que o relatório seja feito focuses on completion of the report, not on who does it. If you want to name the doer, you can add por: ...seja feito pelo João (...be done by João).

Could I replace this with an infinitive, like É importante fazer o relatório...?

Yes, if the subject is generic or understood (like “we/you/one”).

É importante fazer o relatório antes da reunião. = It’s important to do the report before the meeting.
If you want to keep the “that + clause” structure (more formal/explicit), you use que + subjunctive: É importante que o relatório seja feito...

Why is it o relatório and not um relatório?

O relatório refers to a specific, known report (the one relevant to the meeting). Um relatório would mean a report in a more general/unspecified sense. In many workplace contexts, o relatório is natural because everyone knows which report is meant.

What does antes da mean, and why is there a contraction?

Antes de = before (doing something / before a noun phrase).
Here it’s followed by a reunião (the meeting). de + a contracts to da, so:

antes de + a reunião → antes da reunião.

Why is it da reunião and not just de reunião?

Because reunião here is definite: it’s the meeting (a specific meeting). So you use the article a (feminine singular), which then contracts with de → da. If you were speaking generically, you might see something like antes de reuniões (before meetings), but that changes the meaning.

Is relatório always masculine, and reunião always feminine?

Yes: o relatório is masculine, a reunião is feminine. That’s why you have o with relatório, and a (→ da) with reunião.

Could I say É importante que o relatório esteja feito instead of seja feito?

It’s possible, but it’s different in nuance.

seja feito focuses on the action being carried out (the report getting done).
esteja feito focuses on the resulting state (the report being finished/ready).
In many contexts, seja feito is the more standard choice for “must be done.”

Is feito the same as fatto/hecho—why not use a regular past participle?

Feito is the irregular past participle of fazer (to do/make). Many common Portuguese verbs have irregular participles (e.g., escrever → escrito, ver → visto). You simply memorize them; you can’t form fazido in modern standard Portuguese.

How would I pronounce the key parts in Brazilian Portuguese?

A rough guide (varies by region):

É importante: /eh im-por-TAN-chi/ (final -te often sounds like -chi)
que: /kee/
relatório: /he-la-TO-ryu/ (initial r often like an English h)
seja: /SE-zhah/
reunião: /he-oo-nee-ÃO/ (nasal -ão, like a nasalized “ow”)

Can the word order change—like putting antes da reunião earlier?

Yes. Portuguese is flexible with adverbial phrases like antes da reunião. For emphasis you can move it:

É importante que, antes da reunião, o relatório seja feito. (more formal, with commas)
Antes da reunião, é importante que o relatório seja feito. (fronting the time phrase for emphasis)