Every Shabbat we read one parashah from the 54 of the Torah beginning today with Shabbat Bereshit. (Sometimes we double up, reading two parashiyot at a time). We finish up the entire cycle by the end of each Succot.
However, in post-Temple times, the Jews in Israel completed the Torah twice every seven years, comprising two cycles of roughly three years, one slightly longer than the other. Seven years evoke the once-every-seventh-year mitzvah of Hakhel (Deuteronomy 31:10-13), in which everyone had to go to the Temple on Succot to hear the king read from the Torah.
In Babylon, the Jews adopted the one-year Torah reading cycle, which we follow today. While Simchat Torah was an annual event in Babylon, in Israel, it happened only twice every seven years – leading some scholars to believe that its origins were linked to the celebration of Hakhel .