on
D1 – although sometimes you can! – but that the information from D1 is often what goes on to help you solve the game on future days). A few years ago, a long-standing player (I forget which) observed that when Mafiascum towns win, it was typically a consequence of a really high-information D1 elimination which gave a lot of insight into the motivations of the various people on the various wagons (even though much of the time, it was a townie who actually got eliminated).I think it's worth noting that as the game goes on, optimal town strategy and optimal scum strategy become closer to each other (in an extreme case, imagine a 2:1 ending with one player confirmed: the unconfirmed townie and the scum will be doing everything they can to get each other eliminated, so the townie's play and the scum's play should be indistinguishable by that point because they both have identical motivations and the situation is entirely symmetrical). In fact, you can probably make a reasonable argument that the more information that town has, the harder scumhunting becomes, because one of the main scumhunting tools is trying to work out who has more information than they should.
My gut reaction to the question, though, was "the most important day is the day on which lots of players become claimed and people start to get a good handle on what the setup actually is" (this applies primarily to Closed games, of course). Games where town are forced into claiming early often go south quite quickly, for example. Days like that are also a gold mine for gathering reactions, and days
after
that become much less useful for scumhunting because the optimal strategy for each faction becomes a lot more obvious. And of course, it's also the day when you get your "setup speculation hints" (i.e. players who are highly likely to be town, or scum, on the basis of what people have been claiming; this is pretty much an inevitability in Closed setups, and is something that reviewers take into account when balancing games as part of the town's power budget).