A pre-siyum siyum?

Obviously, they chose New Years Day to maximize the number of participants at the ceremonial siyum and achieve the widest positive follow up including brining in new daf yomi learners for the next cycle. Do you have a problem with that?

The Giants and the Jets aren’t playing. But they didn’t know that back then when they had to sign the deal. The stadium is contractually obligated to not rent out the facility on any day that might be a playoff game for either of the home teams.
In general, when the siyum had to be moved because the actual day doesn’t work, the Aguda has always moved it up rather than back. Their stated philosophy was (in the past; I didn’t hear anything about it this specific cycle) that such an inspiring event would cause many more people to want to join. They therefore give them a few extra days for people to arrange chavrusos and/or find shiurim. If they would make it afterwards, and then it takes someone a couple of days to work out a plan, they’d already be holding a few days into Brachos, and people would be discouraging by starting off already in arrears.
[In 1982, the date of the actual siyum fell out during the Aguda convention, Thanksgiving weekend, They made their big bash in the Felt Forum a week and a half before on Sunday. The public explanation was as explained above. They may have also not wanted the event to go head to head with their convention, but they did not say that publicly. My rebbi, Rav Avrohom Chaim Levin zt”l, was honored to say Brachos daf 2 on the day that it actually fell out during the convention. He took that as a siman and a motivation, and he committed to learning daf yomi and stuck with it until his very last day, almost 5 cycles later.]