Sanhedrin 12 - When to Declare a Leap Year
It may be necessary to proclaim a leap year if the grain is too young by Passover, if the ovens for Passover sacrifices became ruined in the rain, or because the Jews in Diaspora started on their journey to Jerusalem but will not reach it on time; additionally if the sheep and goats will be too young by Passover, or if the doves are too tender.
The extra month of the leap year is always inserted before the month of Passover, thus it becomes the Second month of Adar, when Purim is celebrated.
However, a leap year is never proclaimed before Rosh HaShanah, for if it were, people might forget about it by the time that the Second Adar comes six months later. Then they might mistakenly celebrate Passover on the Second Adar and consequently transgress the prohibition of eating leavened bread in the time of real Passover.
Art: Joseph Mallord William Turner - Jerusalem from the Latin Convent