Trabajo cada día excepto hoy.

Because Spanish often leaves out subject pronouns when the verb form already shows who the subject is. Trabajo is the yo form of trabajar, so Trabajo already means I work.

You can include yo if you want emphasis or contrast:

• Yo trabajo cada día, pero mi hermano no.

In Spanish, the present tense is commonly used for habitual actions, just like in English with the simple present.

So Trabajo cada día means I work every day / I work each day. It does not have to mean only I am working right now.

Because cada is followed by a singular noun. It means each or every, and in Spanish that structure uses the singular:

• cada día = every day
• cada semana = every week
• cada año = every year

So cada días is incorrect.

Because cada goes directly before the noun, without an article. In Spanish, you say:

• cada día
• cada semana
• cada persona

Not:

• cada el día
• cada la semana

They are very close in meaning, and in many situations either one is natural.

• Trabajo cada día
• Trabajo todos los días

Both mean I work every day.
A small difference is that cada día can feel a bit more like each individual day, while todos los días is often the more everyday way to say it. But in this sentence, both are perfectly normal.

The accent mark shows the stress and also helps show that día is pronounced in two syllables: dí-a.

Without the accent, Spanish spelling rules would suggest a different stress pattern. So the written accent is important:

• día
• hoy
• trabajo

Excepto means except and works as a preposition here. It introduces the thing that is excluded:

• excepto hoy = except today
• excepto los domingos = except Sundays
• excepto en verano = except in summer

So in Trabajo cada día excepto hoy, hoy is the exception to the usual pattern.

Because hoy normally appears without a preposition or article when it means today.

You say:

• hoy
• mañana
• ayer

Not:

• en hoy
• el hoy (in this meaning)

So excepto hoy is the correct natural form.

Yes, a little. The original sentence is very natural:

• Trabajo cada día excepto hoy.

You can also move the exception to the front for emphasis:

• Excepto hoy, trabajo cada día.

That sounds fine too. But some other orders may sound awkward or much less natural.

Usually, no. In the original sentence, no comma is needed:

• Trabajo cada día excepto hoy.

If you move the exception to the front, a comma is common:

• Excepto hoy, trabajo cada día.

Yes. They can also mean except in many contexts:

• Trabajo cada día salvo hoy.
• Trabajo cada día menos hoy.

All three are understandable and natural. Excepto is clear and standard, and it works very well in this sentence.

It is completely standard Spanish and would be understood across the Spanish-speaking world, not just in Spain.

Nothing in Trabajo cada día excepto hoy is especially regional. It is neutral, natural Spanish.