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Apple is known most commonly to update/introduce products on Tuesdays, hence, today.
 
Not entirely sure what you mean...MacWorld San Francisco in January and Apple's Worldwide Developer's Conference in summer have historically been prime opportunities for new products. Same with Apple Expo Paris, but they've moved away from that in recent years. Other than that, major releases have recently been at special media events scheduled whenever the products are ready for release. Some products (such as last week's iMac/Mac mini updates) see silent updates released on Apple's site with little fanfare.
 
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