Does anyone know if the Milan metro is open 24 hours? Or what the hours may be? We are leaving next Easter morning at 8:00 am from Milan, so of course we'll be needing the metro around 5:00/5:30am. Will it be open then? Thank you.

Just got back from Milan. Don't know the hours, but I can recommend taking a taxi to the train station. Unless you can carry your bags up and down a lot of very steep stairs, take a cab. Not all the metro entrances and exits have escalators.

You say that you are leaving from Milan on Eastern morning, but you don't indicate whether you are flying out, or taking a train. The Metropolitana (subway) in Milan does not go to the airport. In order to get to the airport by mass transit, you have to take either a train (Malpensa Express) from Cadorna Stazione or a bus (Malpensa Shuttle) from Stazione Centrale. I don't know the hours from Stazione Centrale, but the Malpensa Express runs from 4:20 a.m. here's the link:
http://www.ferrovienord.it/webmxp/ita/frame-orari.htm

In answer to your original question, the Metropolitana is closed from about 12:30 a.m. to about 6:00 a.m. here's the link:
http://www.atm-mi.it/eng/oraper/fvnotte.htm