Props to FrodCube for the first part of the work on this - go upvote him.
The sequence, and the next number in it are
28 - the sequence is the number of days in each month, starting in March 2012 and moving forwards
The first mistake is that
many people thought the world would end on December 21st 2012.
"Almost all of you were aware of this wrong answer" - it was widely known.
"even if you knew it was ridiculous" - even if you didn't believe it was true.
Because of the first mistake, we then know that
We're specifically talking about December 2012 for month 10. The following February was 2013 which is not a leap year, so we know that the next number is 28 and not 29.