cellio | daf bit: Sanhedrin 12/13

The Jewish month is lunar but holidays must fall in their proper seasons, so in some years there is a leap month (a second Adar). The g'mara discusses the timing of the decision to add this month -- when do you decide that this year will be a leap year? R. Sh'muel said: the decision cannot be made on (that is, as late as) the 30th of Adar, because that day is eligible to be the first of Nissan and then it's too late. Raba said: consider -- there are 30 days from Purim to Pesach, and from 30 days before Pesach we begin to lecture on the laws of the holiday. If at the end of Adar (two weeks after Purim) Pesach is postponed a month, the people might come to disregard the laws of leaven. (Essentially he is arguing that you can't jerk the people around like that.) (12b)

Eventually on today's daf (13) it will turn out that the decision is made based on how Tishrei falls with respect to summer versus fall -- so months before the current point of contention.