Prepositions of Time

Time is indicated with "a" before months:

Andremo in vacanza a luglio (We're going on vacation in July)
Il mio compleanno è a settembre (My birthday is in September)
Festeggiamo il Natale a dicembre (We celebrate Christmas in December)

Time is indicated with "alle" before hours:

Ci vediamo alle 21:00 (See you at 9:00 p.m.)
La lezione inizia alle 8:00 (The lesson starts at 8:00 a.m.)
C'è il coprifuoco alle 18:00 (The curfew is at 6:00 p.m.)

Time is indicated with "a" before holidays:

Andremo all'estero a Pasqua (We're going abroad on Easter)
A Natale i bambini si svegliano presto (On Christmas the children wake up early)
A Ferragosto andremo in spiaggia (On Ferragosto we'll go to the beach)

Time is indicated with "all" before "alba" (sunrise) and "al" before "tramonto" (sunset)

Mi sono svegliato all'alba (I woke up at sunrise)
Siamo usciti al tramonto perché fa troppo caldo (We went out at sunset because it's too hot)

Time is indicated with "di" (or just with the article) before days of the week that indicate something happening on that day every week:

Di domenica vado a pranzo dalla nonna (On Sundays I go to lunch at grandma's)
Il martedì Franco va in palestra (On Tuesdays Franco goes to the gym)
Di venerdì mangiamo la pizza (On Fridays we eat pizza)

Time is indicated with "di" (or just with the article) before "mattina" (morning), "pomeriggio" (afternoon), "sera" (evening) and "notte" (night):

Di mattina sono sempre in ritardo (I'm always late in the morning)
È bello uscire la sera (It's nice going out in the evening)
Di notte c'è molto silenzio (At night it's very quiet)

Time is indicated with "in" before seasons:

I fiori sbocciano in primavera (Flowers bloom in spring)
In estate le giornate sono più calde (In summer the days are hotter)
In autunno cadono le foglie (In autumn leaves fall)

Time is indicated with "nel" before years and centuries and particularly before the word "weekend". "Nel" is the merger of "in" and "il":

Nel 2006 l'Italia ha vinto i Mondiali di calcio (In 2006 Italy won the World Cup)
Dante è nato nel XIII secolo (Dante was born in the 13th century)
Mi piace rilassarmi nel weekend (I like to relax in the weekend)

Time is indicated with "tra" and "fra" when you need to indicate a limited amount of time to carry out an action:

Ci vediamo fra tre ore (See you in three hours)
Abbiamo una visita tra due giorni (We have a visit in two days)
Fra sei mesi compirò 20 anni (In six months I'm turning 20)

Time is indicated with "tra" and "fra" when you need to indicate a time interval needed to carry out an action:

Saranno qui tra le nove e le dieci (They'll be here between nine and ten)
Gli esami si svolgeranno fra le otto e le dodici (The exams will take place between eight and twelve)
Il check-in è permesso tra le dieci e le due del pomeriggio (It's possible to do the check-in between ten and two in the afternoon)

Time is indicated with "da" when you need to express a determined period of time that hasn't finished:

Siamo amici da 2 anni (We have been friends for 2 years)
Mario vive a Milano da 5 anni (Mario has lived in Milan for 5 years)
La cerchiamo da 3 giorni (We have been looking for her for 3 days)

Time is indicated with "per" when you need to express a determined period of time that has finished:

Ho vissuto a New York per 2 anni (I lived in New York for 2 years)
È stata in ospedale per una settimana (She was in the hospital for a week)
Sono stati sposati per 30 anni (They were married for 30 years)

Note: To indicate a period of time in the past, "fa" (ago) is habitually after the time expressed, but this isn't a preposition:

Siamo andati in Spagna due anni fa (We went to Spain two years ago)
Due giorni fa ha iniziato a lavorare (She started working two days ago)
Si sono sposati un mese fa (They got married a month ago)