You have three options for Milan-Juve. You can buy the cuore rossonero card (15 euros) from the official website, and wait till the sale opens for cardholders. You can take the viagogo route, which is more expensive, but also reliable, and you don't need a card. The third option is to wait for when the sale opens to the general public on the official site, but tickets may be sold out by then.
As for how long it takes for sales to happen, I would guess 2-4 weeks before the game?
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I have a question for Cuore Rossonero card holders. In the past, I haven't needed a card to go watch a game at the San Siro (last I went was 4 yrs ago), but I am thinking of attending a big game this season at the San Siro (Juve or Inter) that will require it. I have ordered the card, and have my number. When the opportunity arises, I would like to book tickets for me and my wife. I know I can book for myself with just the number, but it says that I need to activate the card at Casa Milan or San Siro before I can buy for other people? Is that true? Does she need to have a Cuore Rossonero card as well, or can I buy one ticket for me and one ticket for her when the time comes. I browsed the previous pages in this section, and I couldn't find an answer. Thank you.