Coming out of COVID-19 lockdown: Events, galleries and museums in Milan started opening once again in Feb 2021, but it is wise to check with the venue and book tickets in advance.
DUOMO AND MUSEUM OF THE CATHEDRAL
Opening again: From 11/Feb/2021
Cathedral , Archaeological Area and Terraces
Opening: Monday to Friday
Hours: 10:00 to 18:00. Last ticket at 4:50 pm. Last access at 17:00.
The Terraces are only accessible by lift.
Opening: Tuesday to Friday
Hours: 10:00 to 18:00. Last ticket at 16:50. Last admission at 17:00.
All the info here .