My visit to Milan was restricted by Saturday and Sunday only (6 and 7 December), so I was determined to get 2 tickets for these very touristic days (thanks God it was December, not August). And I've got them. So I want to collect here some useful technical info.

1. Tickets could be purchased (15 euro) on the official site. Every three months they start selling tickets for the next three months. For example, at 30.09.2025 (at local noon, always at noon) they started to sell Tickets for November-December-January. And you do not know exactly in advance when they start to sell tickets for the next quarter. So you need to check the official site periodically to get this date in a few days earlier.
2. I would strongly recommend you to register on this site and fill as many information (payment method, address and so on) as you can in advance. Cause when you select a particular tickets during a booking process they do not reserve them for long: after I put all necessary info during my first attempt, my tickets ("my preccccccious!") were not available anymore.
3. If you have failed to buy tickets on the start selling day you have one more attempt in a Wednesday (noon) of the week BEFORE your visit to Milan. They really provide some additional tickets for every time slot.

But what do I mean "for free"? The first Sunday of every month you can visit "The Last Supper" for free. The only problem - you have to get this free tickets the same way. And this free Sundays' tickets are not available neither three months before nor on previous weeks' Wednesday. They could only be reserved... (surprise!!!) ... on Wednesday of the same week (on 3 December in my case). At noon, surely.

By the way, First Sunday is a holy day for budget tourists in Milan - a lot of museums could be visited for free. Pinacoteca Brera, for example.