Most states have a December 31st deadline for 529 contributions.
However, there are a few states, such as Georgia, that use April 17, 2018 as the contribution deadline.
I've attached a link with the deadlines for all state 529 plans. Remember, you do not need to use your home state's 529 plan. https://www.savingforcollege.com/articles/529-plan-contribution-deadlines-for-state-tax-benefits-877