Answer:
The most important holidays in Italy are those which are related to religious festivals. The biggest of these, of course, are Christmas and Easter.
The Italian 'mamma' or mother is a very important role in Italian society, and therefore Mother's Day, which is celebrated on the second Sunday in May, is a very important time for families to get together.
Valentine's Day is quite big as well, though not as commercialised as it is in, for example, the US or UK.
Each region also has its own 'festas' when local produce is celebrated (anything from potatoes to wine) and singing, dancing and generally having fun are the order of the day.
I have added some links which go into more detail.