The Time Lords at the Earth Orientation Center of the International Earth Rotation Service have again decided the world needs an extra second and have picked New Year's Eve as the best moment for the extra sliver of time.
As the Center's notice states, at 23h 59m 59s on December the 31st you'll need to hold off your celebrations while we slip in the unusual time of 23h 59m 60s before ticking over to 2017 January 1 0h 0m 0s.
The worldwide IT community got a lesson in the importance on leap seconds back in 2012, when Linux's inability to handle the extra moment crashed all sorts of services. Things have settled down since, with only minor leap-second-related SNAFUs occurring in the wake of the mid-2015 insertion.
The advices offer some guidance for each vendor's products, but most of the suggested actions are precautionary. And if rely on either the network time protocol (NTP) or Precision Time Protocol (PTP) the powers that be should have you covered.
But unless you want to end up with a contribution to On-Call, at least glancing at vendor guidance is a fine idea. ®