Why did they start the lantern festival?

After Liu Bang (Emperor Gaozu) died, his wife, the Empress Lv, monopolized the power and ruled the country cruelly, intending to turn the throne to his own family clan (from Liu to Lv). Later, Liu Heng (Emperor Wendi) and other former ministers defeated Empress Lv to get the society back to peace. To celebrate that, Wendi set the date, when the Empress Lv was defeated, as Lantern festival, on which all the family put up lanterns for celebration and entertainment! Since then, the festival was succeeded.