Greetings Julie,
I believe Mr Maria Terence is quite correct about this - the 29th of February 2016 is the culprit here
Go to your Maintenance Plan - Scheduling Parameters tab in IP02 and change the Scheduling key indicator from "Time" to "Time - Key Date". This should make the plan always be called on the same date in the month.
Otherwise, the case is that SAP takes a year to always be 365 days in the scheduling with the "Time" key.
See Maintenance Strategy - Maintenance Planning (CS-AG/PM-PRM-MP) - SAP Library