Alabama State Senator Chris England (D-Tuscaloosa) says that Alabama law requires the Governor to call a special election for U.S. Senate this year, instead of waiting until 2018. See this story. The Governor has already appointed State Attorney General Luther Strange to fill the seat that was left vacant when Jeff Sessions resigned from the Senate to become U.S. Attorney General.
No federal law requires states to hold special elections for U.S. Senate any earlier than the next regularly scheduled-election for congress. The law is very different for U.S. House vacancies than for U.S. Senate vacancies. Each state has its own law on special elections for Senate vacancies.