Actually, AllanM has them backwards. "á" with the mark pointing to the right is an "Acute" accent that happens in longer words such as "água" to indicate the stress, while "à" has the mark that points to the left and is called a *Grave" accent that creates a "Crasis" which is the merging of two vowels or diphthongs into one word (such as "to the" in Portuguese a+a=à).
This is also seen in words such as àquele.
What is tricky about this sentence above is we are supposed to realize that "festival" is a masculine word (it is in the hover-over hints) so if we are presented with an "a" before it then it means, "to" rather than "the" in Portuguese.
To say, "to the festival" it would be "ao festival" but... the other hint that "a" means "to" in the exercise is that DL uses the plural of festival which is "festivais" meaning, to make the sentence translate as, "we go to the festivals" then "the" in this case would be "os festivais" while "to the" would contract to "aos festivals" (clever DL getting us to think and be observant; I like that).
The "a" here is not the definite article, it is the preposition "a" meaning "to" (see: http://dictionary.reverso.net/portuguese-english/a). The sentence "Eles vão aos festivais" means "They go to the festivals" ("festival" is masculine), but see Paulenrique's answer at the top of the page.
A sentence with "will" in it takes it out of Simple Present Tense" and makes it Future tense; in this case, Simple Future which we have not learned yet at the levels that this exercise is placed in the DL lessons which would also change the verbs in Portuguese.
Tell me what the difference is in "going to" and "going to go to".
I do not think this difference, whatever it is exists in this tense for Portuguese.
At any rate we know the sentence exercise is not the same because if you read the other comments here, "festivais" is plural and masculine, so the "a" before it cannot be an article and must be a preposition.
They will go = Eles irão or Eles vão aos festivais
Or, if you read the Tips & Notes in the other lesson module that covers that sentence construction:
A special case
What to do when the main verb is "ir"?
Can I use "ele vai ir" (he will go)?
No, in this case, just use "ir" in present (it will serve both as present and future):
- Ele vai = he will go = he is going to
- Ele vai = he goes