Learn Italian: Le ragazze non si sono mascherate perché ha piovuto tutto il giorno. - The girls didn't dress up because it rained all day.

For weather verbs like piovere (to rain), you can actually use either avere or essere in the past tense. Both ha piovuto and è piovuto are completely correct and mean the exact same thing. Modern Italian tends to lean slightly toward using avere for weather, but you will hear both interchangeably.

If I chose to use essere for the weather here, would the ending of piovuto change?

No, it would remain è piovuto. Even though verbs taking essere normally change their past participle ending to match the subject, the weather "it" is treated as a default masculine singular. So the ending stays as an "-o" either way.

Why is it sono mascherate rather than hanno mascherato?

The verb is mascherarsi (to dress up / put on a costume). Because it is a reflexive verb (shown by the si), it must use essere as its helper verb. Whenever you use essere, the past participle must match the gender and number of the subject. Since le ragazze is feminine plural, the participle becomes mascherate.

Why do we need il in tutto il giorno? Can't I just say tutto giorno for "all day"?