Every Day's A New Day

In Sichos haRan 1 Rebbe Nachman explains that each individual's understanding of God is very personal "כי אני ידעתי" and cannot really be shared with another. Furthermore, even to oneself each day is a different experience of the light of God and a person's experience of one day cannot be understood on another.
Siach Besamim applies this idea to Purim. The Sages instituted five possible days for the reading of the Megilla. There's a day for people who live in open cities, a day for people in fortified cities and a few days for villagers. זמנו של זה לא כזמנו של זה. If a person would read the Megilla on a day that is not appropriate for where he lives then he has not fulfilled his obligation. This is because each individual's experience of the Purim story and his appreciation of the miracle that occurred is affected by where he lives. Different populations experience it differently. Therefore they each much read it on a unique day. And even if one experiences it fully on the day that is appropriate for his locale  he can't possibly celebrate on the following day when he no longer has the fullness of yesterday's experience.

Siach Besamim is a work written by Rabbi Rephael Rosenman in which he follows a common Breslov style by connecting each idea in Sichos HaRan to Purim. He has a similar work on Pesach as well as seforim that connect Likutei MoHaran to both Purim and Pesach