Some reasons why Tuesdays are "must" instead of "good" would be way more helpful then a simple statement "they said so". So all I can say is: Yes, let's target Tuesdays if that is idea (which I agree), but if there is a slip then slip only a day, two or three if the problem can be fixed within that timeframe (which for example was not the case for the first release slip for the final, but maybe for the second).
Setting aside the fact that my gut tells me that most slips will require about a week to fix->rebuild->retest->restage anyway, here's an answer:
Fundamentally, it's about putting your news out at a time when the most eyeballs are going to see it. Red Hat's press machine has been working with the Fedora Project Leader and the Fedora Marketing team to maximize the attention that is cast on Fedora for a long time now, and it is their recommendation (not their order, but their recommendation) that we try to have our most important news be released on Tuesdays. If you look back at our history, we've kind of been all over the map in terms of day-of-the-week, but settled down in recent times.
Theory states that the more proud you are of your news, the earlier in the week you want to put it out. Mondays and Tuesdays are the best days for that. My guess is that we go with Tuesdays as a hedge against all the news that tends to come out on Mondays (so we don't get buried), but so that we can also build momentum the entire week with interviews, and have opportunities (as we have taken in the past) to announce a few different Fedora or community-related pieces of news all in the same week.
Finally, consistency is important. If Red Hat's PR team knows that Fedora is coming on a Tuesday, it helps them to plan their schedule so that Fedora can have the day all to itself. Even if we slip, knowing that slips only go from Tuesday to Tuesday is still helpful.
I must respectfully disagree with the person who said this:
I think it's a bad idea to delay the releases more than necessary for marketing reasons. IMHO we should release as soon as technically feasible.
I think it is perfectly acceptable, and responsible, to factor a marketing plan into a product's schedule, up to and including the day of release.