Should I expect the majority of the shops to be open or closed on Monday 28th December in Milano?
The majority of shops will be open on Monday 28th of December. The reason being that Monday is neither a typical closing day, nor is the 28th a public holiday. This is especially true for large shopping centres and brand stores. Some might have shortened their opening hours during the school holidays, although usually this occurs during pre-festive days such as the 24th and 31st. For example La Rinascente is open with modified opening hours during December.
It is worthy to note that the establishments which are customarily closed on Mondays, regardless of school holiday periods, will be closed on the 28th. These include barber shops, and other small family-run businesses.