Learn Arabic (Modern Standard): ستبدأ العطلة غدا.

The prefix سـ (sa-) is added directly to the beginning of a present tense verb to turn it into the future tense. Here, it attaches to تبدأ (it starts) to mean 'it will start'.

Could we use سوف instead of the prefix سـ in this sentence?

Yes, you can! You could say سوف تبدأ (sawfa tabda'). Both سـ and سوف express the future tense. سـ is an attached prefix often used for things happening soon, while سوف is a separate word that can sometimes imply a slightly more distant future, but they mean the same thing here.

Why do we use the feminine verb تبدأ (she/it starts) for a vacation?

In Arabic, non-human objects are still treated as masculine or feminine. The word for vacation, العطلة, ends in a taa marbuutah (ة), making it a feminine noun. Because the subject is feminine, the verb must also be in the feminine 'she/it' form (تبدأ) instead of the masculine 'he/it' form (يبدأ).

Why does the verb come before the subject in this sentence?

Arabic often uses a Verb-Subject word order, which is different from English. While you could technically say العطلة ستبدأ غدا (Subject-Verb), starting the sentence with the verb is the most standard and natural structure in Modern Standard Arabic.