Why Italian says 'a Natale' and 'a Pasqua' with no article
When you say when something happens on a holiday, Italian uses the little preposition a followed by the bare name of the holiday: a Natale (at Christmas), a Pasqua (at Easter), a Capodanno (at New Year), a San Valentino (on Valentine's Day). There is no article, so no al and no alla.
English speakers often get this wrong because English changes its preposition from holiday to holiday: at Christmas, on Valentine's Day, over Easter. Learners then reach for in, su or a contracted form like al. In Italian it is much simpler: one preposition, no article, every time.
The construction is always the same: a + holiday name. Italian treats these names almost like the names of cities, which also take a with no article (a Roma, a Milano). The holiday works as a fixed point on the calendar, and a simply means "at that time."
Because there is no article, a never merges into al, allo or alla. It stays a short, separate word.
Rule of thumb: whatever English uses (at, on, over), Italian uses a and nothing else.
Capodanno is New Year's Day, but a Capodanno often covers the whole celebration, including the night before. San Silvestro is New Year's Eve, December 31st, and works the same way.
Pasqua is feminine, so you might expect alla Pasqua. Italians don't say that: it is just a Pasqua, like the others. San Valentino also takes plain a, even though English says on Valentine's Day.
carnevale is usually capitalized when it means the festival season before Lent, and you'll hear it in the famous saying a Carnevale ogni scherzo vale (at carnival, anything goes).
Other Italian holidays follow the same pattern, for example a Ferragosto (on August 15th) and a Ognissanti (on All Saints' Day).
Avoid the typical mistakes: al Natale, in Natale, su Natale and alla Pasqua all sound wrong when you mean "at Christmas" or "at Easter."
Per is also correct, but it adds a meaning: "for" or "by" the holiday. It is very common with gifts, plans and deadlines.
The rule only covers the bare holiday name. As soon as you add a word like scorso (last), or use a phrase like la notte di or il giorno di, the article comes back. It also returns when the holiday is the subject of the sentence: Il Natale è la mia festa preferita.
Personal celebrations are not holiday names, so they take the article: al mio compleanno, not a mio compleanno.
To say when something happens on a holiday, use a + the bare holiday name, with no article: a Natale, a Pasqua, a Capodanno, a San Silvestro, a San Valentino, a Carnevale. Use per when you mean "for" or "by" the holiday, and bring the article back when the name is modified or is the subject of the sentence.