The first deduction holiday for 2017 is already here. Because deductions for health & welfare occur 4 times per month, months with 5 pay periods produce a "deduction holiday". December 2016 had 5 Saturdays, so the first deduction holiday of 2017 will be Jan 10th. Because of open enrollment changes a higher number than usual are in "catch-up" status. See link below to FAQ - health & welfare deductions - for more details.
On the other hand, retirees can elect medical and dental coverage together, or just medical coverage.