Ta'anit 18 Prohibiting Fasting on Celebratory Days

There are two fast days - Adar 13 and Adar 12 - that the rabbis argue we should not fast and we should not eulogize on.  They tell stories about these two days.   My understanding is that these two days in Adar are to be celebrated, and thus we are prohibited from afflicting ourselves at the same time.

13 Adar was know as Yom Nicanor.  Nicanor was a Greek general who would wave his arm at Jerusalem and claim that one day that land would be his.  Nicanor wills killed by invading Romans.

12 Adar is known as Yom Trayanus.  Trayanus was a Roman officer who had two Jews killed.  The following day, he was removed from his post and killed by his own superiors.

The Gemara discusses Ta'anit Esther, which is considered to be different from other fasts.  We celebrate the Jewish victory over Haman on that day.  Ta'anit Esther is used to explain a couple of different concepts - both of which are still fuzzy for me.  The concepts consider what should or should not be done the on the day before a fast day.  There seems to be some tension between observance the day before a special day and observance on that special day itself.  

One of the more confusing - and interesting - factors at play is that our halacha changed over time.  The observance of Yom Nicanor changed; the observance of Purim changed in response.  Our traditions might seem static, but they are in fact alive and pliable.  It is the community that determines how rigid we want our halachot to be.  With learning, thoughtful consideration and consultation, our halacha must reflect the needs of our changing communities.  Today's daf reminds us that this has been done numerous times over the course of the past 2000 years.