Learn Spanish (Latin America): El próximo fin de semana voy a viajar a la montaña. - Next weekend I am going to travel to the mountain.

This uses the near future formula: a conjugated form of the verb ir (to go) + a + an infinitive verb (a verb in its basic, unchanged form). Here, voy means I am going, a connects the verbs, and viajar is the infinitive meaning to travel. Together, they mean I am going to travel.

Why are there two a's in voy a viajar a la montaña?

They are doing two completely different jobs! The first a is a required part of the future tense formula (ir a + infinitive). The second a is the regular preposition meaning to, showing your destination (a la montaña means to the mountain).

Why do we say el próximo fin de semana instead of just próximo fin de semana?

In Spanish, when you talk about specific dates, days of the week, or the weekend, you generally need to use the definite article (el or los). While in English we just say next weekend, Spanish literally says the next weekend.

Semana is feminine, so why does the phrase use the masculine word el?

The main noun in the phrase el próximo fin de semana is actually fin (end), which is masculine. The part de semana (of the week) just describes the end. Because fin is masculine, it takes the masculine article el.

Where is the word for I (yo) in this sentence?

Spanish speakers often drop subject pronouns like yo because the verb itself already tells you who is doing the action. The verb voy can only mean I am going, so you don't need to say yo voy unless you want to add extra emphasis.