I expect Gov. Bentley of Alabama will appoint Sessions’ seatholder until the 2018 election.
Alabama law specifies that:
a.) The Governor will appoint an interim Senator.
b.) Then, a special election will be held on such a day as the governor may direct, unless vacancy occurs between 2 and 4 months before the next regularly-scheduled general election, in which case it is held at that election. If vacancy occurs within 60 days of the next regularly-scheduled general election, a special election must be held on the first Tuesday after 60 days have elapsed since the vacancy occurred.