The origin of reading a portion from the prophets in synagogues
The Shabbat and holiday Torah reading service in synagogues is followed by a recitation from one of the books of the prophets and is called the haphtarah, or haphtarot in the plural. The name means conclusion,” and is so called because it follows and ends the Torah service. The Hebrew Bible is called Tanach, which is an acronym for the three parts of the Bible: Torah (the five books of Moses), Neviim (the Prophets) and Ketuvim (the Writings, such as the book of Psalms).
The haphtarot are selections from the middle group. The earliest report of the practice of reading from the Prophets on the Sabbath in the Synagogue is found in the first century New Testament book Luke 4:17–20. The first mention that a portion from the prophets was read after the Torah reading is in the second century Mishna, Megillah 4:1. However, we no longer know when the practice of the prophetical recital actually began.
Many scholars believe that the custom started in the second century BCE, during the Maccabean period when the Jews were forbidden by the Syrian-Greek king to read the Torah in the synagogue. Jews circumvented the decree somewhat by reading a selection from the Prophets that corresponded in some way with the contents of the Torah portion. This reminded the people of their long-standing tradition to read the Torah in the Synagogue on the Shabbat and holidays and reminded them of the contents of that Shabbat’s portion. Thus, for example, the portion of Tazria deals with the laws of leprosy, so a story from II Kings 4:42–5:19 was selected to be read since it tells the story of the leper Naaman.
When the Maccabees defeated the Syrian-Greeks and Torah reading was restored to the Synagogue, many scholars believe that the people decided to continue the new tradition of reading the haphtarot; however they placed the reading after the more significant Torah portion.
 A version of this explanation appeared in my book “A Rational Approach to Judaism and Torah Commentary,” published by Urim Publications.