Learn French: Je pars en Espagne vendredi. - I am leaving for Spain Friday.

Because leaving for Spain is a one-time event happening this coming Friday. If you added le to make it le vendredi, it would mean it is a recurring habit, like saying you leave for Spain every Friday.

Je pars is in the present tense. Can we use it for a future trip like this?

Yes! Just like in English, French frequently uses the present tense to talk about confirmed, scheduled plans in the near future.

Why do we use en before Espagne?

To say you are going to or are in a country, French uses the preposition en if the country is feminine. Most countries that end in the letter e, like Espagne and France, are feminine.

I noticed vendredi starts with a lowercase letter. Is that correct?

Yes, it is perfectly correct. Unlike English, French does not capitalize days of the week or months of the year unless they are the very first word of a sentence.