Gino raised several questions about the calendar....

The first day of the month, in ancient times, was called with the first sliver of the new moon was observed.  This would normally mean that the moon would be full on the 15th day of the month.  It could be a day off or so because there is only about a 1 or 2 hour window in which the moon can be seen as new right after the sun goes down.  If it turns new JUST AFTER that window the new moon won't be called until the next day, so the phases are more mature than in a month where it turns new right after sundown.  So it could  vary by a day.  But normally, the full moon will be the 15th of the month, and Sukkot (Tabernacles) and Passover will be on a full moon.