Learn Italian: Ieri è piovuto, ma i bambini hanno voluto mascherarsi per il Carnevale. - Yesterday it rained, but the children wanted to dress up for Carnival.

Verbs about the weather, like piovere (to rain) and nevicare (to snow), are flexible! In modern Italian, you can use either essere or avere as the auxiliary verb in compound tenses. Both è piovuto and ha piovuto are perfectly correct and mean the exact same thing.

Why is the auxiliary avere used in hanno voluto mascherarsi if mascherarsi is a reflexive verb?

When a modal verb like volere is followed by a reflexive verb, the choice of auxiliary depends on where you put the reflexive pronoun. Since the pronoun si is attached to the end of the infinitive (mascherarsi), you use the regular auxiliary for volere, which is avere (hanno voluto). If you moved the pronoun to the front, you would use essere and match the gender and number: si sono voluti mascherare.

Why is there an il before Carnevale when English just says 'for Carnival'?

In Italian, holidays and festive periods often take a definite article, especially when following a preposition like per or when referring to the festival in general. You will usually see per il Carnevale or per il Natale, though Italians sometimes drop the article (per Carnevale) in casual everyday speech.