Learn Italian: È nevicato molto e non ho visto né i carri né le maschere. - It snowed a lot and I saw neither the floats nor the masks.

Verbs that describe the weather, like nevicare (to snow) and piovere (to rain), are special: they can use either essere or avere as their helping verb in compound tenses. Both è nevicato and ha nevicato are perfectly correct and mean exactly the same thing.

If weather verbs take essere, why doesn't the ending of nevicato change to match a subject?

When a verb takes essere, the past participle usually has to agree with the subject's gender and number. However, weather verbs are impersonal because there is no real subject doing the action, just a general weather "it". With no specific subject to agree with, the participle defaults to the masculine singular ending -o.

Why is there a non before the verb if we are already using né ... né (neither ... nor)?

Italian requires a double negative here. When né ... né comes after the verb to mean "neither ... nor", you must put non before the verb. You are literally saying "I did not see neither the floats nor the masks".

I know carro usually means "cart" or "wagon". Why is it translated as "floats" here?